As long as no other program installed can control the clock speeds of the GPU, like an OCing program, I would say the card is bad. To test this try the GPU in a different machine and see if the issue still occurs. IF the issue still occurs then replacement is in order, whether it is an RMA or purchase of a new one.
If the issue does not happen on a different machine then there is a program causing this to happen and that will take some diagnosis on your end to find.
At this point what I would do is test the card to be sure the card is good and go from there.