Help troubleshooting system crash, please.

Hey everyone,

I'm looking for some help trying to troubleshoot random system crashes on my month old homebuilt system.

First I'll give you my specs, then describe the problem, then tell you what investigations I've done so far.

Specs:

MB: GIGABYTE GA-P55M-UD4 LGA 1156 Intel P55 Micro ATX Intel Motherboard
GPU: Geforce 8400gs
PSU: CORSAIR CMPSU-750TX 750W ATX12V / EPS12V SLI Ready CrossFire Ready 80 PLUS
CPU: Intel Core i5-750 Lynnfield 2.66GHz LGA 1156 95W Quad-Core Processor Model BX80605I5750
HDD: SAMSUNG Spinpoint F3 HD103SJ 1TB 7200 RPM SATA 3.0Gb/s 3.5" Internal Hard Drive
RAM: G.SKILL ECO Series 4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800)
OS: Win 7 Enterprise

The problem:

I've gotten three random system restarts with no warning this week. It has only happened when I have multiple applications running, including a game. The system simply restarts with no warning (I thought it was a power outage the first time).

Investigations so far:

My first thought was an overheating problem. I am not currently overclocking. So, I tested with SpeedFan.

Baseline temps:
System: 25C
Cores: Avg 32C Highest 35C
GPU: 50C

After stressing with Prime95 for 15 minutes:
System: 43C
Cores: Avg 50C Highest 54C
GPU: 50C

These temperatures don't seem high to me at all, what do you guys think?

My second thought is that it might be a PSU problem. I'm inexperienced with dealing with these problems so I'm asking here. The +12V number on my SpeedFan has been acting very strange and jumping around. Its baseline is less than 1V, but when stressed with Prime95 jumps to 9V.

The Corsair 750W PSU should be plenty of juice for this minimal system, no? So, unless its just faulty, I don't see the problem. I can go get a voltameter down at Radioshack and test the voltages directly, but I've never done this and wouldn't know where to start.

What recommendations do you have on what to do next if this problem continues?

Thank you,
Stumbles