1.5 year old system randomly shuts off

Hiya everyone. Sooo...the history...I built my first system a year and a half ago, using these forums for help along the way in both deciding which parts were best and in the actual building process. Everything worked great and I had my dream computer, it seemed, up until about two or three months ago. That's when the random shut down's started. At first it was only once every other day or so..now it's several times a day and at completely random times. Stress isn't a trigger...I can be running a game, a movie, some music, uploading pictures, and browsing the web all at once and it'll stay on just fine...and other times it'll shut off. Other times it could simply be on the desktop with absolutely nothing running and it shuts off. No warnings, no flashing lights...just completely random like the cord being pulled out of the socket. Bought a new surge protector and plugged everything into that, then moved it all to a different outlet, which didn't fix anything. Next thing I did was install some software to monitor the temps of everything (AIDA64), thinking maybe I had a over-heating issue. Ran some stress tests and found that the CPU, at 100% usage over 5 minutes never went above 47C (across all 4 cores), the video card never went above 50C, and the mobo stayed around 32C. Even after seeing that, I went ahead and reapplied thermal grease and bought an extra case fan for the last empty slot I had. No change, still got random shut offs. Next I tried running a bunch of virus/spyware/malware scanners...nothing...then I defragged and did a disc cleanup (even though those are done once a week anyways). Then I tried taking everything apart and putting it all back together...still nothing. The last thing I tried was switching the memory sticks around, and into different slots...and yet it's still randomly turning off. I'm at my wits end with this and could really use some advice on what to do next.

Specs:
CPU Type QuadCore AMD Phenom II X4 945, 3000 MHz (15 x 200)

Motherboard Name Asus M4A78T-E (2 PCI, 2 PCI-E x1, 2 PCI-E x16, 4 DDR3 DIMM, Audio, Video, Gigabit LAN, IEEE-1394)
Motherboard Chipset AMD 790GX, AMD K10

2x 2gig DDR3 Patriot memory sticks

ATI Radeon HD 4800 Series (1024 MB) video card

Disk Drive SAMSUNG HD502HJ ATA Device (500 GB, 7200 RPM, SATA-II)

Optical Drive TSSTcorp CDDVDW SH-S223L ATA Device (DVD+R9:16x, DVD-R9:12x, DVD+RW:22x/8x, DVD-RW:22x/6x, DVD-RAM:12x, DVD-ROM:16x, CD:48x/32x/48x DVD+RW/DVD-RW/DVD-RAM)

Rosewill XTREME Series 850W PSU

Windows 7 Home Premium 64-bit OS


Thanks for taking the time to review this, I know it's really lengthy but I wanted to make sure that I gave as many details as I could.


*Edit: I forgot to mention that nothing is overclocked, it all runs at stock. I was always afraid to overclock since it voids warranties.