Problem with computer

Hey everyone. I have been having problems recently with a newly built computer. I have ran a couple games on it before and i always seem after a good 30 mins of play the screen freezes and the computer locks up. I can't Ctrl + Alt +Del or anything except reset the computer. I am posting this in hopes of receiving some feedback on some troubleshooting tips on how to diagnose this problem. I want to apologize in advance if this has been incorrectly fourmly posted. I am going to post all my comp specs. I have ran a memtest and everything came back fine, I have ran 3Dmark and got a score of 8k. I really dont' know what could be wrong. Any feedback in appreciated.

Windows Vista Ultimate
AMD Phenom II X4 940 Processor
OCZ SLI 4096MB PC6400 DDR2 800MHz (2x2048MB)
Asus M3N72-D nForce 750a SLI AM2+ Motherboard
Seagate 750GB Serial ATA HD 7200/32MB/SATA-3G
Ultra LSP650 650w Power Supply
EVGA GeForce GTX 260 Core 216 896MB PCIe

As mentioned the power supply could be at fault, or it could be a temperature issue since it takes 30mins for the problems to begin. Keep an eye on your temps and see if you notice anything as your using your computer.

Could be heat, memory, power or BIOS problems among others. Temperature and memory problems need to be addressed first.

Check your BIOS and make sure the memory settings match your memory's specifications.

Please download Prime95 and CoreTemp. Run Prime95 on all cores (with "Detect rounding errors" checked) for at least one hour, watching core temps. Let us know what happens. That will tell us about heat and provide another type of memory test.

Download from:
http://forums.tweaktown.com/f69/la [...] ols-30530/

That error usually means a memory error. The error can still be the RAM stick(s), mobo, heat, or psu. The fact that core temp did not go over 61C should eliminate heat.

Check your BIOS and make sure the memory settings match your memory specs. If BIOS says "Auto", use CPUID's CPU-Z to get the timings being used, and CPUID Hardware Monitor to get the voltage. Record what it says, then manually set BIOS to your mem spec.

Rerun the test using 1 stick of RAM. Then run the test again using only the other stick of RAM.

Sorry lol. I should know better by now how to make that clear.

Download CPUID's "Hardware Monitor" from the same site I gave you above. That program will display your actual memory voltage.

Sorry again - I didn't realize an AMD cpu might not show there. Try AMD Overdrive instead.

That's pretty strange. Did you ever install the chipset drivers?

Unfortunately, I'm not an AMD guy. If someone else doesn't come along soon, I'd suggest you start a new thread asking for help after getting an error running Prime95.