Is that 5.0 MHz or .5 MHz on the OC? I would recommend going back to original settings on the GPU, then start the OC in increments of tenths, one at time, (example; 3.0 original, increase to 3.1) then check how the GPU works while gaming. If that increase works well then do it again and see how it goes until it becomes unstable again. Also when overclocking make sure you bump up your fan speeds to counter act the heat build up on the GPU as well as your case fans to help all-around case cooling.