DAW on the FRITZ - next Step

Hello.  Six months ago I built a new DAW to run Win 7 64 bit and Sonar X2a (DAW specs at the bottom of the post).  Since then I have been dealing with intermittent crashes/freezes.
 
I am now writing to ask what my next steps should be in fixing this.
 
The problem: the computer will occasionally and randomly "freeze" - it simply stops responding to the mouse and keyboard: Ctrl-Alt-Delete does nothing.  To regain control I have to shut the computer down and reboot.  Because the freeze/crash involves the OS, there is no blue screen, no error messages, no error log or crash log.  I have seen these crashes mostly when using Sonar, but also when using the Win Media Player, and when mousing around on the Desktop.  So I don't think this is a software problem.
 
What I have done previously:
1) Deleted and re-built the AUD.ini file - continued to get freeze/crashes
2) Deleted and re-installed the audio interface driver - continued to get freeze/crashes
3) Replaced the StarTech firewire interface card with one from SIIG (having a TI chipset) - continued to get freeze/crashes
4) Fully updated the OS - continued to get freeze/crashes
4) Ran MemTest+86 (ver. 5.01) - zero errors were reported
5) I have 2 8gb memory modules in this computer: I tried running the computer with each module separately.  I continued to get freeze/crashes with both memory modules installed, and with each module installed individually.  Given the negative MemTest, and these results, I think I can be pretty confident that this is not a problem of the physical memory.
6) Took the DAW apart and unplugged all connections to the MOBO and components and then put it all back together again, in the hopes that there might be a loose or incomplete connection causing the problem - continued to get freeze/crashes.
 
So now what do I do?  My thoughts:
1) I could uninstall and re-install Sonar, or perhaps better uninstall and re-install the entire OS and all software.  Given that I get these freeze/crashes in a variety of situations, I don't think this is a software problem.  So I think this is just wasted effort.
2) Replace the MOBO
3) Replace the processor
4) Replace the power supply
5) Replace MOBO, processor, and power supply - obviously I don't want to replace the MOBO if the problem is the power supply, and it is pointless to replace the power supply if the problem is the MOBO.  Given that I still don't know the exact cause of the freeze/crashes, this option is the most likely to fix it, but also the most expensive and labor-intensive.
 
Can anyone advise me on the best way to proceed?  Is there anything else I have overlooked?
 
DAW specs:
I built the DAW specifically to run Win 7 and Sonar X2. The computer has no other functions. The specs are as follows:
Processor – Intel i5 3570 quad core @ 3.40 gHz (normal clock – NOT overclocked)
MOBO – ASUS P8Z77-VLX w/ Intel Northbridge Z77 Express chipset
Memeory - 2 x Corsair Vengence CMZ16GX3M2A1600C10B (2 modules of 8 GB apiece: total 16 GB), DDR3
Power - Corsair CMPSU-750TXV2, 750 watts
Firewire - StarTech.com 4 port PCI 1394a  firewire card - replaced with SIIG DP 2-port firewire card
OS - Win 7 Home Premium 64 bit, SP-1
Cakewalk - Sonar X2a (I believe I have all Sonar X2 updates installed)
Audio Interface – Mackie Onyx 1604 w/ firewire card installed (driver ver. 1.7 installed – is the only driver version Mackie makes for this device and this OS)
MIDI Interface – MOTU Micro Express USB midi interface (Motu “universal” driver installed – is the only driver version Motu makes for this device and this OS).
 
Thank you

Hello.  Good thought, bvideo.  I have checked the temps in the processor, and after an hour or so of use, the temps are around 58-59 degrees C, well within the operating range or the processor.
 
I haven't really thought about the HDs as the cause of this problem.  Usually, a HD problem manifests itself in different ways (i.e the computer won't boot to the OS if the "system" drive has a problem, or if it is a non-system drive that has a problem, the OS won't recognize the drive)

+1 
A stress test is likely to crash the system if there is a hardware error, and if it crashes with something like Prime95 running and Sonar out of the picture altogether, you can stop concentrating on a Sonar issue. 
 
The problem with saying that Sonar crashes your DAW intermittently, is that most likely your DAW is going to be running Sonar when it crashes, because that is what you spend time running. Unless you are willing to spend hours running other software to see if the crash is really Sonar specific, you are not going to be sure. 

Oh - the tech support people at ASUS suggested I try clearing the CMOS, and updating the BIOS.  So yesterday, I completed both clearing the CMOS procedure and successfully updated the BIOS.
 
I am doubtful doing this will resolve the freeze/crashes, but will see how the computer works for the next week or so.