i would say if underclocking it somehow fixed it, thats a sign that your card is possibly defective, i've had that issue before and unfortunately had to get it sent back and replaced.

if you have access to 3dmark, you could try to run a stress test and see if your card performs ok as well, that could tell you if your card is running ok or not. than you would at least know if its just simply a weird issue with the game running under your specific configuration.