Computer Randomly Restarting

My problem first started about a week ago. Bought the World Of Warcraft game and installed it. Major lag. Noticed that my computer was only seeing 256 of the 512 ram i was running. Fixed the problem no lag. I upgraded my memory from 512 to 768. While in game just at random times my computer will restart completely, no errors, a couple times i recieved a BSOD. Took back the new stick of ram and got it replaced, still the same problem. Thought my CPU might be over heating replaced the heatsink. my CPU runs at 42C idle. and just less than 50C on a heavy load. Still having the same prob. Replaced my powerunit from 350W to 500W. Still same problem.

My Specs:
AMD Athlon 2400+ Xp
Ati Radeon 9600 Pro 128MB
768 pc 2700 ram
40 gm HD

My case has 6 80mm fans counting the two for the power unit.
my video card fan works fine. very clean
my CPU fan works without a hitch.
I'm at the end of my rope.. anyone know whats going on here?

 

yes the 256 that came with the computer is still there.. and i have tried switching them all around both sticks of 256 and the 512, trying everything. also i have updated my video drivers, the WoW patches update automaticly or i couldn't play. My sound card is just the onboard card... not sure if i need to update those. and i will check the memory as soon as i get home tonight.. i'm at work...

 

In Control Panel/System/Advanced, uncheck Automatically Restart in the Recovery area. Then when it crashes you will see the cause message.

 

Man when i got home from work last night first thing i did was check the CPU temp. because it had been off for about 15 hours.. the CPU temp at startup was 22C. Then i did the MEMCHECK thing.. ran through that test without a hitch. Then i went into MSCONFIG and turned off all startup programs (i read this somewhere) and turned on my game.. played for about two hours without a restart.. i'm starting to think that it had something to do with one of my startup programs. maybe the drivers were incorrect. or something stupid like that... also as i was going to be i turned on prime95 and let it do it's torture test for 6 hours.. this morning i woke up and stopped it, it stated that it had ran for hours with no errors, and no warnings.. that test uses 100% of the CPU for the 6 hours, and the CPU temp was still 50C when i got up.. so i think i might have found the problem.. anyway.. thank you guys for all the help...