Crashing with no errors

Having intermittent crashes were my computer powers itself off and restarts itself, just like hitting the reset button. There are no error messages, No BSOD, no recovered from critical error and no memory problem.
It has done it rarely when I am on desktop or using the internet, maybe 3 times in 6 months.
Worst is when I am playing games that are so so for graphic needs, min specs on the one game is at least a 64MB video card. WHen I am playing it can crash and do the reboot once to several times a day or for however long I am playing.
My system is a year old, never had many problems untill that last 6 months maybe.
Specs are:
RocketFISH 700W PSU
Phenom II X4 940 Black edition (not overclocked)
8GB DDR2 1800 RAm, (think its 1800, its the highest you can go for DDR2)
Asus M4A88T (I think, not positive) Recent Bios updated
ATI Radeon HD 4850 512MB
WD 1GB internal and WD 1GB external (OS on internal)
Stock heat sink and fan that comes with the Phenom Core
Lots of space in my case and have 2 120MM fans and a smaller 80mm fan going.
Core Temp.exe tracks my average temp at 49C when idle and average 56C when gaming
Never had a heat issue with it,

Is there a way for me to find out if there is an error that is causing my computer to crash like it is, once I have a handle on that error code (if there is one) then i can fix it up. If not I would most likely start buying parts and peices until i either A, have a new system or B, replace a faulty something or other.
Never had an issue with anything crashing. 700W should handle everything. Video card is like two years old but hasnt given me so much as a burp. Same goes with the processor.

On another note, when in game the fans really kick into high gear it seems and arent so quite as when I am not playinh

Realism,
I've been experiencing similar symptoms as yourself...It would either freeze while I am playing starcraft2 or moderwarfare2 however the durations were random and my temps didn't go above 40C so I know it's staying plenty cool...Also if it didn't freeze it would simply randomly restart like you described. I looked into the windows logs which revealed nothing just a "critcal" error due to system shutdown. I don't think these problems started until I installed the service pack 1 for windows so I'm wondering if that may be the culprit. I've read multiple posts suggesting changing memory timings in the BIOS but I've fine tuned my RAM to no avail....I've also heard theories of disabling the windows sideshow but this didn't work either. I'll keep searching for a solution and keep you posted with my findings, hope you do the same!

-Elmo