Sacalait
Sorry to hear you're having some issues.  I'm using X3e and while I occasionally experience something I'm encountering no showstoppers.  But as some have said I'm not using all the functionality in Sonar.  I treat it more like a multi-track tape recorder than anything.  Are you creating a groove clip when you're looping?  I do it all the time and I can't say anything goes out of sync.  This works best when doing it in 4 or 8 bar increments.  

 For me it's lots of showstoppers :( 
 
I also use v basic functionality but use the Pro Channel. 
 
The looping issue involves looping a section of the timeline with soft-synths playing back. it slowly slips out of sync after looping for a while. No idea why. this issue has been around for ever though. Can't believe it hasn't been quashed by now

There are a few posts on looping and workarounds. IIRC, they bottom-lined on the metronome needs to be engaged (but can take the fader to zero to remove the sound). This seemed to correct the timing drift issue.

Another was disabling count-in, but I believe that applies to MIDI recording only.

Googling with "site:forum.cakewalk.com" before the key words is the best way to search in here.

Good advice as usual from Mettelus. FWIW I'm pretty sure the looping is now fixed.
 
Really I think there are two issues: Stability and cost of maintenance/updating.
 
Windows is highly cost-effective but only if tweaked properly, and the causes of issues are not always obvious. Even if you have a clean computer, install Windows 7, then install SONAR and use only 64-bit plug-ins Windows can still be problematic due to interactions. It took me years before I realized how much the "HD Audio" codec installed by my graphics was impacting performance and limiting the ability to get low latency. It is extremely frustrating to find out something like having a floppy disk port enabled on your motherboard by default but no floppy drive present can cause problems. It's also frustrating that the more a program depends on Windows for performance, the more likely there will be problems. Often cross-platform programs can be more stable, but at the expense of some level of performance hit.
 
As to maintenance and upgrading, having been raised in the days of tape I have a somewhat different perspective. A four-track recorder with nothing else cost me $2500 adjusted for inflation, and I had to keep dishing out at least $50 per month for tape. The option was renting time at some studio that had probably $100K-$250K worth of gear. Today's DAWs, including SONAR, do pretty much everything those studios could do from a recording standpoint (e.g., excepting acoustics, room treatment, etc.) and for me, spending $150-$200 a year to maintain SONAR in peak condition isn't that much more than replacing my strings every few weeks. Also remember that while many updates are about bugs within SONAR, many are also about issues caused by interactions with other software or Windows itself.
 
None of this is to minimize the frustration that happens when your tools aren't working right, I've "been there and done that" and it sucks. However, it's what I tell myself when I need to remember that all things considered, running a computer-based DAW is a big improvement over what I was dealing with 25 years ago. I'm very grateful my system runs so smoothly and I can count on SONAR to get the job done, especially since the upgrade to Platinum. But, I attribute a lot of that to the hardware platform.

Do you run any other software that puts a similar load on the PC to Sonar? If so does that software behave itself?
 
I'm also a little confused by the RAM you're using. The Amazon link you gave says that particular RAM is a 3 SIMM set intended for use with Intel Extreme Edition cpus in a triple channel capable motherboard.
 
However, as far as I can tell from Intel's website, your i5 is not an Intel Extreme Edition and supports two, not three, memory channels. I've also looked at the manual for the Gigabyte P67A-UD5 (revision 1)  which says the board supports dual, not triple, channel RAM and for dual channels RAM should be configured as two (two in channel 0 or two in channel 1) or four identical sticks.
 
Otherwise, though the manual doesn't say so, with an odd number of SIMMS the board will be running the RAM in single channel (quite a bit slower) mode. The BIOS might also not be reading the RAM's XMP data correctly. Whether this might cause RAM problems I don't know, sometimes it will and sometimes it won't. If you're getting general instability issues, such as Windows crashing, you could try removing one stick of RAM and making sure the other two are in the correct slots to work as a dual channel pair. Then go into the BIOS and check the memory speed is correct there. You may even need to slow the RAM down a little, RAM manufacturer's claimed specification and the real world capabilities of their products sometimes differ :-/ RAM that the manufacturer claims to be e.g. 1333 but capable of overclocking to 1600 may be fine in their test system at that speed but PCs vary so much that may not be the case in yours.

I went through this before, and it drove me crazy. I assume that you listed the ram at amazon because you updated it yourself. Double-check the ram CAS latency, and make sure that it matches what it says in the BIOS. Don't rely on auto-detect. Put the actual figures in the BIOS. I've had several friends go through the same thing, and it was usually a CAS latency ram issue.
 
http://en.wikipedia.org/wiki/CAS_latency

Re tlw and Resonant Serpent: there sure are some smart people here.

Don't know about smart, more like having gone through the "what the ****** is wrong with this PC build now" enough times.

My big wake-up with RAM issues came with a PC I put together when Vista came out. The RAM was on the motherboard approved list and it's makers said it should run at <whatever the speed was>. BIOS picked it up "correctly" but as soon as Windows loaded it was freeze or blue-screen time. So I ran memcheck off a boot disk for 48 hours straight. No problems found. Booted into Vista and.......crash. Throttled back the RAM and things worked very well and still are, only that PC's a Linux server now.

Conclusion? Windows isn't only an operating system, it's also one of the most brutal RAM testing applications on the market. Any hardware problem and Windows will find it. To be fair to Microsoft and Windows application coders, it doesn't help that PCs are so variable in their hardware it's almost a miracle they work at all.

I'd see if the same issues exist in Reaper since you have it. 

tlw
My big wake-up with RAM issues came with a PC I put together when Vista came out. The RAM was on the motherboard approved list and it's makers said it should run at <whatever the speed was>. BIOS picked it up "correctly" but as soon as Windows loaded it was freeze or blue-screen time. So I ran memcheck off a boot disk for 48 hours straight. No problems found. Booted into Vista and.......crash. Throttled back the RAM and things worked very well and still are, only that PC's a Linux server now.

Conclusion? Windows isn't only an operating system, it's also one of the most brutal RAM testing applications on the market. Any hardware problem and Windows will find it. 

My conclusion would be it isn't a memory issue at all. If it's working in Linux and passing memcheck AND it's happening straight after windows boots it's more likely display drivers, or the firmware on the graphics card (or GPU onboard the motherboard). I just had to go through a similar issue which was resolved by updating my graphics card BIOS, it was not fun. Or it could be some other piece of hardware that needs drivers or firmware updating, or even chipset drivers.

SilverfoxUK
 
Thanks, 
 I'm using a Roland Cakewalk Interface (VS-100)! drivers fully updated. I have also disabled onboard audio devices. I haven't updated the chipset drivers since first built the DAW machine. i'll look into this. 

Make sure the VS-100 is started after windows has booted not before. Make sure the VS-100's firmware is updated. e.g.System Program Version 1.50 http://roland.com/support/article/?q=downloads&p=VS-100