Need to know the motherboard.
I'm feeling it's either cpu temp or VRM throttling.
It's an EVGA nForce 750i SLI bored.
Here's a link, except mine is not the FTW edition. http://www.evga.com/articles/401.asp
I believe this is the manual for my specific board: http://jp.evga.com/support/manuals/files/122-yw-e173.pdf
I'm assuming the FTW OCs relatively better than the standard SLI edition.