PC won't turn on / stay on

My computer first started powering off like at the end of a shutdown (all fans and drives spinning down) around a month ago, in the middle of the night. The next night it happened again, and then in the course of my work the next day it died again, during non-intensive use (just some web browsing/OneNote). I set it in a corner for a few weeks, and then when I plugged it back in it lasted for a few days straight before dying in the middle of some City of Heroes. I got progressively less time (around a few hours) with each restart until the fans etc. would simply twitch when I pressed the power button.

A week or so before the trouble started I was seeing BSODs when trying to S3 Suspend to RAM (not sure if it ever worked as I didn't usually suspend much and I think it might have been on S1 when I tried) so I switched to S1 in BIOS settings and was able to suspend successfully. Also, the first night after I set my computer back up, I tried to shut down: it stayed on the black screen after fading out for ~15 minutes before bringing up a BSOD complaining something about the ACPI. This has led me to believe the problem is with the motherboard's ACPI controller, although naturally when I called the motherboard manufacturer they blamed the RAM (despite my point that if the issue were with the RAM the BIOS would at least be able to start). Any insight as to what could be the root cause of this problem? It's not overheating, as several times when this happened I was not doing anything intensive and when the BIOS came back up I checked the temperatures in setup and they were all below 60C.

Here are my system specs:

Windows Vista Ultimate (x64) SP1
GA-M57SLI-S4 (rev 2.0), BIOS ver. FF
AMD Athlon 64 X2 4400+ Brisbane 2.3GHz Socket AM2 65W Processor Model ADO4400DDBOX
OCZ Platinum Revision 2 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop Memory
XFX PVT80GGHE4 GeForce 8800GTS 320MB
SAMSUNG 20X DVD±R DVD Burner Black SATA Model SH-S203B
Ultra 500-watt SLI-ready power supply
3 hard drives (2 ATA 1 SATA), a floppy drive, an internal card reader. Sorry I can'r really give more specifics except that the floppy drive is integrated with the card reader.