It could be any of 3 things:
1) Temps are too high and causing the GPU to throttle/crash
2) The voltage is too low for the higher clock speeds, causing a crash
3) PSU can't supply enough power to the system.
It is most likely number 1 or 2. What are your temps under load? If it's overheating, put the fan at 100% (if you can handle the noise) or lower clock speeds. Also if your card had unlocked voltage, try raising it a bit (.025), or if it's locked, lower the clock speeds.