Comp Crashing/No Solid Explanation

Well i'm puzzled over this one... My computer has random crashes and restarts and ive tried many things and i still cant pinpoint what the issue is. here are the specs of my comp,

CPU - E8400
MB - xfx GeForce 9300 INTEL 775 DDR2 Motherboard
VC - xfx 9800gt
RAM - CORSAIR XMS2 4GB (2 x 2GB)
HD - Seagate 1tb
PSU - Zumax 650w
OS - Windows 7 32-bit

ive heard about the 9800gt's overheating but ive replaced the heat sink with a good aftermarket one and the charts show its about 45c idle, 50-52 when running games. The games are not even graphic intense either, for example, i'll be playing the original half-life on GoldSource engine and it will crash. I have run memory test, burn in pro, 3dmark and other tests and everything seems to be in working order.

90% of the time it does not give me a BSOD, just a frozen screen and im forced to flip the psu switch. The other 10% i'll see a blip of a BSOD and thats it.

does anyone know of any issues with anything else that might be a problem?
if anyone knows of any issues with anything to help please let me know

 

Did this just start happening?
Any hardware changes made to the computer recently? Any driver changes?

What is GoldSource engine? Not familiar with it.

Sam

 

Are you getting any memory dumps? See this link.

http://forums.majorgeeks.com/showthread.php?t=35246