hi, i thought i would share some info on this for others:
XPS 8100 running WIndows 7 64bit purchased around July 2010.
A few weeks (or months?) ago, I would get random crashes, usually when left overnight. No indication of what the problem could be. (Note: I never saw a BSOD it just always said "entering power save mode" on the monitor. Seems the computer keeps running for a bit, probably writing out the dump file). The crashes became more frequent until this week it was crashing almost every time under 1 hour. I booted in Safe Mode and let it sit and it still crashes. I did the F12 diagnostics plus some other hardware diagnostic utilities and no errors were detected (although during one very long memory test, it crashed in the same way).
I pulled the panel off an dusted out the inside with a vacuum. I then reseated all the memory chips. Still crashes. Read some more in the internet and suggested to clean and reseat the video card (Nvidia GT220). AS I was removing the video card, I noticed how extremely hot it was. Too hot to touch. The computer had only been on for about 10 minutes so it seemed quite unusual. Once I let it cool and got it out, I dusted it out really well, including running some q-tips around in the fan area. Lots of black dust in there. Got it satisfactorily clean and put back together. Turned it on and let it sit. It sat for several hours without any crash now. I'll give it a long term stress test today/tonight.
I downloaded a program to monitor CPU heat, voltage, etc and everything is running at reasonable temps. The GPU is running at 41C, which is obviously much cooler than before (too hot to touch). I wish I had measured the temps before the cleaning. I think it's a good idea to periodically check the temps. I'm going to start doing that.
Anyways, seems the problem was due to an overheating video card. I hope that helps someone.
I'll post more if anything else comes up. Happy to answer any questions too.