My computer freezes at random intervals when I'm using it the screen will freeze and the light on my G5 mouse and keyboard will turn off, and I'll have to restart the computer. This happens less when I'm surfing the net, and more when I'm playing a game, for example i only got to the loading screen on L4D and it froze. I've never had the computer not freeze at some point when I've played a game, which usually takes 15 minutes or less I've run memtest on each stick of ram individually, and they both passed. I recently replaced my 8800 GTX with a 9800 GT because the former broke(my fault), and I've reformatted my computer 5 times this week, with the problem persisting, any ideas on whats wrong?

Case: 3D Aurora 570
Mobo: Gigabyte GA-P35C-DS3R
CPU: Intel Core 2 Quad Q6600, 2.4ghz
Ram: Corsair 2gb PC6400 5-5-5-12
Graphics: XFX 9800 GT
HD: 500gb
Pwr: Antec True Power Trio 650 Watts
Screen: 2 Samsung SyncMaster 204BW

I did that, when it crashed, the temps were as follows
Core #0: 55
Core #1: 51
Core#2: 49
Core#3: 54
Doesnt seem like its the CPU thats overheating, if anything is
Forgot to mention that when I leave the computer off for an extended period of time, like overnight, the first time I turn it on, the computer will not post, when i push the reset button, it will post and starts up normally, maybe this is related?

its about 2 years old, and I think the freezing has been going on for a while, but its gotten worse, I replaced the graphics card only a couple weeks ago. I've ran memtest, and the sticks passed, but I'm not sure how long to run it. Curiously, even though my ram passed, on my system monitor (some little addon from google desktop) even at idle, it says my memory is at 50% usage, which I think is strange, but then again, Im not sure how reliable the information it reports is. If it is a power problem, could I do anything to test whether this is the culprit besides getting a new PSU?

If I look into the bios w/ some utility, what utility would I use and how would I know if a voltage is off?

I got speedfan and cpu-z and basically crashed my computer by playing L4D a bunch of times (I hope thats what you meant by stressing the computer). The Vcore voltage was at around 1.2 like you said and when I stressed it, it remained between 1.1 and 1.26V. I took a look at speedfan and the readings for V core 1, Vcore 2, +3.3V, +5V were all pretty stable. +12V, however, started at around 2V at idle, but jumped to 4V when I was stressing it. Also, after my computer crashed, I saw a message that says "windows has recovered from a serious error" twice, but I'm not sure if that means anything, just trying to give as much information as I can about the system, sorry if I've been giving too much.

yes, I'm looking at speedfan, and the +12V reads 2.56V right now at idle, while the -12V reads -16.97V.

well now, I have a more pressing problem, I updated my BIOS, as suggested by my friend, and now the computer starts up, but will not POST, nothing shows up on the screen, and all I see is my fans spinning. I've flashed the BIOS and tried restarting several times but that didnt work. Now I'm stuck...

Glad to hear that it's working now. Looks like a bad DIMM, I guess. Sometimes those memory errors are so intermittent that even a thorough diagnostic such as MemTest86+ can't find the problem. I'm still curious about the funky voltage readings. Were you able to check in the BIOS to see what the voltage readings are there?