I've got a bit of an odd issue with my computer, which has been going on now for the past year or so..but it happens so infrequently and I am having such a problem tracking down the cause that i've not really been worrying about it that often. It has happened a few times this week however, and its just clicked that maybe one of you brainy people could help

It is basically a random lockup/freeze. I will be using the computer, and suddenly I won't be able to move the mouse any more, and any animation/moving pictures on the screen will just stop and freeze. If there is any audio playing it will start to slow down and stutter, finish playing whatever is in the buffer and then just start buzzing. I could be doing anything but it seems to happen more when I am just at my desktop in Visual Studio, outlook or browsing the net (any browser) - however those are the things I do the most on this computer so I suspect that what I am doing is unrelated. I have also seen it happen when I have left my computer alone doing nothing (say to make a brew), and I come back to find it locked up.

I have been quite lucky never to lose any work etc from it (I am a control-s addict lol), and it doesn't seem to corrupt any data, but its getting annoying!

This is my current hardware specification:

Mac Pro (early 2008 model)
1000wt Apple PSU
2x 2.8ghz quad core Xeons (E5462 Harpertown)
4x 1gb PC2-6400 FB DIMMS (all Micron Tech/official apple ram)
1x Nvidia 8800 GT 512
1x Nvidia 8500 GT 512
4x Hard drives (1tb, 500gb, 320gb, 80gb - all samsung)
Windows 7 Ultimate (final, 7600)

I think thats all the specs that matter. I should mention that the same problem also happened when I only had the 1 graphics card in the system, so I think it is fairly safe to rule that out. I have also had the same issue in multiple OSs, including Vista, various versions of 7 and also server 2003.

I have tried using each of the above hard drives as the boot/OS drive (currently using the 80gb as its the fastest), and all present the same problem. Swapping the ram around to different sockets has made no difference, all sorts of drivers combinations has made no difference, and the system passes various hardware tests (including a full 24 hour apple hardware test), again with no problems. All the hard drives report that they are fine and no issues with S.M.A.R.T or bad sectors that I can find.

For some reason I have not run memtest yet (??) not sure why as thats surely an obvious one hehe..so I will do that after writing this post. The big problem with finding this issue is that it happens so infrequently that I can't give it to Apple to have a look at under applecare as they will take it for weeks while they try to reproduce..and I really can't be without this machine for that long. I also cannot confirm whether the same thing happens under OSX at the moment, without messing about installing VMWare or parallels to be able to work on my machine (unless theres a hidden VS port for osx somewhere )..however as I suspect hardware to be at fault I don't think that would help anyway.

Any ideas as to what it could be, other than the ram which I will check shortly? Or any other tests that you can reccomend? Any ideas/suggestions are most welcome!