Over-clocking and under-clocking are messy processes, I'd set the graphics card back to factory defaults and consider reinstalling whatever operating system you use after a reformat of your HDD (obviously back everything up before this).
Typically hardware failures from the GPU or CPU you'll get a BSOD, so it seems more likely that it's not a hardware fault, or at least not a typical hardware fault.
Oh also update all your drivers, doesn't hurt.
Unfortunately I don't know any good diagnostic tools or such, but if you could get any more information on the problem that would certainly help.