I'm a computer technician and what you describe seems unlikely to be an actual problem.
My guess is that the "slow down" is natural for the game with your card for that area.
Since you describe the 10FPS slowdown dropping to the same rate as an OC'd version of the card then I can only assume that the game is not being limited by your graphics card at that point.
While the GTX680 can change its frequency on demand in a game, your card can not. It will operate at full processing capacity unless the bottleneck is elsewhere in your system.
FYI, some monitors can operate at 1920x1080 @ 50Hz so you can VSYNC at this slightly lower refresh to avoid screen tearing.
My advice is slightly drop your quality settings to ensure a more constant 50FPS or 60FPS synch rate but I don't believe you have anything physically wrong.
Well, I am not running with v-sync. As I understand, the card has 3 settings, idle, entertainment, and gaming. When running games, it's supposed to stay at my maximum OC and not slow down. It didn't do this before, that is why I am kinda worried.