If *not a single thing in your entire computer* is overclocked (other than your GPU factory overclock)...try underclocking your GPU to reference card specs/speeds.
If that fixes your problem, RMA your graphics card, it's unstable at the clock speeds it was shipped with.
If it doesn't fix the problem the list of suspects grows to include just about everything else. But it still sounds like general instability. (CPU, RAM) (although a PSU that cant keep voltages high enough, could also cause instability if/when voltage "droops" too much.