The program you are using to adjust this overclocking is the issue. uninstall, restart, and reinstall. This happend to me with my evga software. it was reading 705mhz on my gx 670.
also, cross reference with a different program that measures the mhz, temp. to make sure its just not the program getting a bad read.
if its still doing it, uninstall your drivers.. restart, then reinstall.