Hi,
No dropouts or crashes here - X2a has been quite stable for me.  I primarily do audio tracking, but also use some soft synths and tons of plugins for effects.  My latest projects typically have about 50 tracks or so. 
Russ

Mine crashes quite frequently, but I believe it's due more to the old and cobbled together system I have than with Sonar. Once I start nearing 25 or 30 tracks with lots of plugins Granny starts to wheeze and eventually wets the bed. I'm getting pretty good at improvising workarounds, though.
All that said, I'm an enthusiatic X2 fan.

X2a seems to be pretty stable here. I’m not using many 32 bit plugins and I usually bounce midi tracks (Mainly Kontakt 5) to audio before recording anything else.

And please forgive thee irrelevent aside: MelMeyers, I remember you from your days in Tulsa, when you were working with KMOD, and I was playing with Bliss. Very cool to see you here.
End of digression. carry on.

gswitz
I don't get crackles and pops but I do sometimes crash Sonar.

I was seeing lots of little pops -- maybe 15 of them in a 3 minute stretch of recording one track to audio.  I thought maybe there was a problem with my mic or audio i/f.  They appeared randomly, not at punch points.
 
I don't think I have had this problem since I started using the FX disable button before recording.  I wish there were a way to designate some effects as being persistent, though.  I hate disabling the limiter.

Windows 7 64-bit professional, Intel i5 Sandy bridge 3.5 GHz CPU, 8 GB RAM, 2x SATA HDDs, Nvidia GTX 460 gfx (latest 'stable' drivers).  2x Creamware AKA Sonic Core Pulsar II DSP PCI soundcards (one provides the I/O) w/ latest 64 bit drivers.  Sonar Producer 64 bit X2a.
 
I would say that my entire system has been showing some instability signs of late, but Sonar has not crashed as a result of this.  Often I'll get hangs/crashes *after* I exit either Sonar or (especially) the Sonic Core application.  I strongly suspect:
 
o Nvidia software, not necessarily the drivers but all the other crap that comes with them.  I got rid of their 'Nvidia Experience' application recently, it certainly was 'an experience' using it.
 
o Firefox, I should uninstall it really.  IMO this browser application is going from bad to worse with its resource hogging and inefficiency.
 
o Java. Can't stand it.  Garbage.  I think I've finally purged my system of it now.  Horrible, inefficient and a massive security risk.
 
o Maybe the LeapMotion drivers &/or M$ Security Essentials.  Less likely, but they both really drag the system down at start-up/boot time.
 
I've got too much s/w on my computer really, it's not a 'dedicated DAW'.  I'd like to dedicate a machine to that purpose at some point but for now I use it for everything.
 
But anyway, Sonar has been very solid of late.  No crashes and it seems to be behaving itself when looping very well.  I think it still gets confused with orphaned notes but I look out for those now and manage them 'manually'.

cparmerlee

gswitz
I don't get crackles and pops but I do sometimes crash Sonar.

I was seeing lots of little pops -- maybe 15 of them in a 3 minute stretch of recording one track to audio.  I thought maybe there was a problem with my mic or audio i/f.  They appeared randomly, not at punch points.
 
I don't think I have had this problem since I started using the FX disable button before recording.  I wish there were a way to designate some effects as being persistent, though.  I hate disabling the limiter.

Take a look at the plugins your project is using - (the project with the pops and crackles and snorts and such).  It sound like you may be using one or more plugins during recording that were really meant to be used POST-recording (for mixing and mastering).  Some plugins are very CPU-intensive, and others have 'look-ahead' processing, and those 2 kinds of plugins are intended for mixing/mastering.  please list all of the plugins used in this particular project, and we can get to the bottom of this.  The easy fix is to swap out those effects for others - just until finished with the recording, and then you can swap back to the ones meant for mixing/mastering, and your dropouts should go away.
 
These plugins seem to be reported as causing cracks/props/dropouts during tracking: Concrete Limiter, LP-64 Multiband Compressor, LP-64 EQ and Perfect Space Reverb.  If any of these are loaded into the project at the time of recording, you may trigger problems with the audio (Rice Crispies - snaps crackle and pop, or dropouts).  So, again, swap out those plugins for reasonable replacements - JUST UNTIL FINISHED WITH RECORDING - and then you can put them back in when you are ready to mix/master.  Please note that there may be additional plugins that also cause these kinds of problems (possibly the Boost 11 Peak Limiter and Transient Shaper, and maybe others).
 
Hope that helps, please review the above, try swapping out plugs listed above, and post back results.
 
Bob Bone
 

No crashes here. I limit 32 bit plugins as much as possible. Use lots of softsynths and effects. I was involved in a thread recently involving multiple instances of (Sonic Charge's excellent) Microtonic. Microtonic by default sets up with MIDI out enabled - I had always used it as a single instance in a project - multiple instances, each with MIDI out enabled, caused repeatable crashes. With MIDI out disabled in the plug, no crashes.
For those who aren't having fundamental system (hardware) problems - eg - with high or spiking DPC latency - but who are using a lot of synths, and/or tempo-based effects, etc, I do think there is still a fundamental weakness in SONAR's MIDI routing implementation for softsynths - that could be contributing to crashes or general instability. If this is you, check if you have plugs that are sending MIDI out that needn't be, or if multiple instances are contributing to instability where a single instance isn't a problem.
It's been brought up before but obviously is still kicking around as an issue. 
The other observation is that X1 and X2 are a lot more graphically demanding than their predecessors. Some plugins - all of the AAS plugins are good examples of this - cause gui-related issues, and can create hang-ups, slow response, etc. I use (and love) all of the AAS plugins in most of my projects and I have learned to keep their gui displays closed or minimized except when I need them, or eventually I'll get into weird display issues, and have to close the project and restart it. 
Food for thought - nice to see some constructive input - 
 

X2a:  no crashes, no drops outs.  Works like a charm.
 
I am running an HP, off the shelf computer with no tweaks or adjustments of any kind. The computer has no software installed except for X2, Band In A Box, and whatever came with it.  It is not hooked up to the internet.
 
Win 7 Home
AMD 2.8 GHz processor
64 bit
8 Gig RAM
Many VSTs and VST instruments
Interface:  Tascam US600

X2a crash free here, too. Has been a rock, and I use it professionally (unless I'm scoring to picture....in which I use Cubase 7). 
 
Custom Build PC
i7 2700K running Win 7 Pro
32GB Ram
3 x HDD + 2 SSD's
Audiophile 2496 PCI soundcard.
700W power supply
S++tload of VI's
Tim Horton's