my best guess is you aren't using the latest miner or a non-compatible one (or you are way overclocked).
Use Claymore's miner:
https://bitcointalk.org/index.php?topic=1433925.0.bat file will be:
setx GPU_FORCE_64BIT_PTR 0
setx GPU_MAX_HEAP_SIZE 100
setx GPU_USE_SYNC_OBJECTS 1
setx GPU_MAX_ALLOC_PERCENT 100
setx GPU_SINGLE_ALLOC_PERCENT 100
EthDcrMiner64.exe -epool eu1.ethermine.org:4444 -ewal <Your_Ethereum_Address>.<RigName> -epsw x
all of this is from the ethermine.org page:
https://ethermine.org/ETA: pick the server closest to you
North America (East): us1.ethermine.org:4444 or us1.ethermine.org:14444
North America (West): us2.ethermine.org:4444 or us2.ethermine.org:14444
Europe (France): eu1.ethermine.org:4444 or eu1.ethermine.org:14444
Europe (Germany): eu2.ethermine.org:4444 or eu2.ethermine.org:14444
Asia: asia1.ethermine.org:4444 or asia1.ethermine.org:14444