Screen Goes Black During Gameplay, No Video Output

[boiler1990]

Here's my setup:
Core i7-930 CPU
EVGA FTW3 SLI-certified Mobo
Antec 300 Case with 5 fans (2x120mm hard drive intake, 1x120mm side intake, 1x140mm top intake, 1x120mm rear outtake)
2 EVGA GTX 460s (SLI, OCed)
Antec Earthwatts 650W PSU

I got my 2nd 460 on Thursday this past week, and never experienced this problem with the GPU OCed and at 100% usage.
I use MSI Afterburner to OC the cards' core/shader/memory clocks to 850/1700/2100 MHz (also used these settings when I only had 1 GPU).

While I'm playing any game (happened on multiple old and new programs) on the highest settings for about 20 minutes, my game hangs for a second or two (not just graphics, sound does as well) and my monitor goes black and says there's no video input. Everything inside my computer is still running, but I can't control anything.

I think this may be an overheating problem, but the fan speed is set to Auto in MSI Afterburner, but after extended periods I can't hear an increase in fan speed. Also, when I reboot after this issue, Afterburner reads temps around 50C, but I don't think the temp is exceeding 70C, since 1 card only reached 60 once. I can't check it since I lose all graphics capabilities after this problem occurs.

I would appreciate any insight and suggestions.
Thanks!

 

[sdf]

Sounds like it could be a PSU issue, maybe one of the lemons that gets through from time to time or does not actually output the listed wattage for dual 460's.