It's probably hitting it's thermal limit when a load is applied and it's locking up instead of just kicking you to the desktop. When you look at the sensor tab of gpu-z at an idle your speeds are in a 2D mode which should be around 1/2 of your overclock. When you actually try to go into 3D performance mode I bet your idle temps are shooting up to the 70d+ range before it actually starts doing any real work.