I've had this card for about a year now, and it has started doing this sort of thing lately. I've overclocked it up to 950 MHz at one point, but dialed it down to 900 when it started getting a bit too hot. I did some burn in test's with FurMark, but it never went above 90 degrees. Restarting my PC usually fixes it for a while, but then it refuses to go above 405 after maybe an hour of gaming. all my other components are running fine, though the CPU is at maybe 45 degrees idle, since it's slightly overclocked and it still has a stock fan.
This happens sometimes when you overclock too much. The card is downclocking itself so it wont get damaged and sometimes it gets stuck. I would say either lower your overclock a little more or keep the card a little cooler or or just live with it and reboot when this happens. A reboot will clear things up until what ever has been causing it to downclock hits again.