Unstable PC - lock ups / restarts while under stress
I'm scratching my head over this one.
Motherboard: Gigabyte GA-H77M-D3H
GPU: Gigabyte ATI HD 7850 OC Edition PCI-E 3.0
CPU: Intel Core i3 3220 3.3GHz Cache LGA 1155
PSU: FSP Raider Series 650W 80 Plus Silver
HDD: Western Digital Caviar Blue 1TB 64MB 7200rpm HDD SATAIII
OS: Windows 8 64-bit
I'm experiencing restarts on my uncles PC when the GPU/CPU is under stress (Movies, Gaming etc). This has been happening since day 1. I've had overheating issues before, but the temperatures are well within acceptable ranges when the restart / lock up occurs.
The motherboard had a few issues with it to begin with, so that has been recently replaced (which I thought was the problem, but unfortunately not).
I have tried his GPU, PSU, RAM, and HDD on my machine (running on my motherboard) - all of which work together flawlessly. I have also tried running my GPU, PSU, RAM, and HDD on his motherboard - which works fine. This leads me to believe his CPU could be the issue, or a combination of something.
Does anyone have suggestions to check before I dismantle both of the CPU / Heatsinks and swap them over? (I don't really want to if I can help it!)
Thanks in advance!