Well I occasionally experience occasional freezes with a third party video card. What I found was that the latest display drivers freezes my system and so I can't really ever update them. How I managed to nail it down was by removing the card and then never getting a lock up.
 
I'm suggesting doing the same thing but in reverse in order to rule out a display driver issue. Hopefully you've updated your display drivers by now (via Intel driver update or otherwise, not sure about your display chipset).
 
Thanks..

BbAltered
....  I can sort of reliably cause a crash problem by starting the computer, launching Sonar, load up a project, and then walk away from the computer.  When I return to the computer after 60 minutes or so, typically the computer is crashed/frozen... 

This is particularly telling.  Could there be a memory leak somewhere?  Reboot, start up a fresh session and monitor memory usage in task manager.  If memory is not the culprit, what task(s) are eating up cpu cycles?
 
Make sure you can't point to a specific plugin causing the crashes.   Leave them entirely out of the project (Just audio).  Does it still crash after awhile?
 
Disable all of the unnecessary processes running in the background, including those in the BIOS.  (Refer to one of the DAW optimization guides for what to disable).
 
Make sure your DAW's power profile is set to "High performance".   There are important additional settings under Advanced - make sure everything is set to never turn off.
 
That's enough to start with.  Good luck!
 

Hello JonD.  Thank you for you thoughts.
 
I have updated Windows, Sonar, got the latest MOBO drivers, updated the MIDI and audio interface drivers, and updated the BIOS.  I have tweaked Win 7 using all of the standard tricks that audio computer gurus suggest - including disabling hibernation and setting the power settings to "High Performance" and setting display and HDs to never sleep - and a whole bunch of other tweaks as well.
 
As for memory, I have run MemTest+86 for two FULL cycles - with no reported memory problems.  In addition, I swapped memory modules in and out of the computer on the off-chance that there was a bad memory module that MemTest was not identifying. 
 
And Sonar still crashes/freezes.
 
The last time I got a crash/freeze, I had a new (empty) project loaded into Sonar; no plug-ins running, no audio or MIDI data in the project, and was using the Media Browser to find some audio files to import - and then the computer froze solid.
 
What is telling for me is that I get no crashes/freezes when running Reaper - recording audio, recording MIDI data, editing audio and MIDI, running plug-ins, etc.  If there is a problem with the memory or a "memory leak", it does not occur when using Reaper.  This tells me that the computer components and audio and MIDI interfaces all work perfectly well.

>  This tells me that the computer components and audio and MIDI interfaces all work perfectly well.
 
Nope I'm really really sorry to state this again, Sonar will not freeze up your machine and it doesn't make other peoples machines freeze either unless they have dodgy drivers/hardware/underlying OS fault. The OS would have captured the event in the event viewer, and if the fault error was at application level the OS would have terminated the application cleanly. Other software may work but this in no way shows that this is a Sonar issue or that hardware works perfectly well, different software can use the drivers in different ways.
 
In regards to faulty memory and CPU it's probably best to put it on an overnight burn in test for 12 hours to fully test it, two cycles may not be enough. However you have other suggestions here to follow, I'm very keen about the additional display adapter suggestion myself as display drivers seem to be a major cause of freeze ups, and there are loads of other suggestions in this thread now from me and others.
 
It is important to understand Sonar is not freezing, Windows is freezing, drivers or OS fault will trigger that not an application. Sonar may be calling that buggy driver or some other background app, or maybe even the OS itself (i.e. dodgy virus scanner hooking into the OS or actual virus on system).

Thanks and good luck..

Hey AlexS. 
 
I think we can fully rule out a problem with the memory modules themselves.  I have run the computer with a couple of different memory modules in it - swapping modules in and out of the computer; I continue to get crashes/freezes regardless of which module or what combination is in the computer.  So unless multiple modules have the same defect (none of which are being detected when tested by Memtest), we can be highly confident this is not a problem of the physical modules themselves.
 
And again, no crashes/freezes when using Reaper - and if there is a problem with the memory modules themselves, we would expect that problem to show up when running any software.  And that is not happening.  So I am highly confident that the memory modules themselves are a-ok.
 
I will make a deal with you regarding the display adapter: I will buy any display adapter you suggest, install it and run it in my computer.  If that solves the problem, wonderful!, and I give you a great big thank you.  If the problem continues, YOU reimburse me the cost of the display adapter, the cost of shipping, and one hour of my time.  I should point out that whatever problem exists with the current display set-up does not manifest itself when running Reaper or any other audio software I have.  So I am thinking this is barking up the wrong tree.  But you are very confident about this, and if you are willing to accept some of the risk with me (keep in mind that there is a possibility a different display adapter could cause some OTHER problem - as you yourself have found out), then I am willing to give it a try.  Agreed?

You can pick them up second hand for a few dollars on ebay or walk into any computer mart which will have a cheapo card. Or why not borrow one. I will pay you as long as you accept my consultancy fee whether it works or not which will be at least double the cost of the card and your time ;)

Btw I worked for Samsung once building 100's of custom boxes for customers and the way we tested the hardware components was burn in memory/CPU/graphics tests for 24 hours. It does work. You are probably right it isn't memory but you can't be 100% certain on two passes that's all I'm saying. It could also be PSU,CPU,motherboard,hard drives etc etc etc as well. In my experience display adapters or GPU, PSU, memory or dodgy CPU are the most common cause of OS freeze ups. Motherboards as well. Display adapters and memory are the two worst cases followed by PSU. Often drivers or firmware solves most issues and if no dice you need to look at swapping out components.

Anyway once again good luck...

Hello AlexS
 
So what is your recommendation for a display adapter - Make, model?

Anything anybody can lend you or you can get cheap. I really can't really recommend one above the other there are lots of opinions out there.. Some say AMD are crap and some people say nVidia is. I don't think it's a brand issue myself, more like how good the specific adaptor is. Please await other opinions from forum users, or google reviews, I certainly won't recommend my current card.

Btw Did you run Intel update/update your GPU drivers? Check for firmware updates everywhere etc. Do that first.

http://forum.cakewalk.com/FindPost/3159682

You have 3  choices right now. 
1.You can believe your computer is at fault
2.You can believe X2 is at fault
3.You can believe the Mackie Drivers are at fault.   
 
1. I doubt it, you sound like you have looked into all reasonable issues. 
2. I'm not sure why Alex feels he needs to defend an obsolete, and buggy version of Sonar. It is high on my list of probabilities. 
3. I would ask around and see if the Mackie is working for others under Windows 7 64 bit. 
Try other web sites like Gear slutz. You really need to talk to other users. Is there a forum for the Mackie? 
 
 

Just out of curiosity is Reaper, Sound Forge,and NI Komplete (in stand-alone mode) configured to use ASIO drivers?