I had a similar problem with what I thought was my 680 lagging out.
What was really happening was that it was constantly throttling between idle and max.
It first started to appear when I started to shut down my computer at night (saving $$$). Two out of three times I would start it up my games would constantly seesaw between fluid buttery goodness and 10fps lag fests. It was definitely a software issue on mine though. If I went into the nvidia control panel and pressed restore and then apply, my screen would blank out for a second and everything would be fine until the next reboot.
Through a lot of uninstalling and reinstalling I can say with some (66%) confidence that having a possibly corrupted EVGA's Precision X starting with windows was the culprit as a clean wipe and install of that program was the final step that brought everything back stable. (for now)
(the above is a reposting of what I said in another nvidia earlier today)