Hmm.. one possibility is that it's going into hibernation when your CPU is drawing alot of current, causing the voltage at the power supply to drop below the threshold, initiating shutdown/hibernation. Can you try doing something very CPU and/or GPU intensive to see if you can cause it to shutdown? Maybe by running a 3d game?
If that causes hibernation/shutdown, then I would look into:
1) finding a better ground
2) upgrading power and ground wires to thicker gauge wires