It usually helps if you provide more information such as which GPU, what config parameters used, which version (Genoils or ethminer 0.12.x) etc.

Anyways, possible issues:-
1. You have set farm-recheck to 200? If so dial it down to 2000
2. You have overclocked the GPU? If so, dial it down a bit... Suggest you try without any over clock for a few hrs before overclocking.