What could be wrong? I'm mining it with CPU and it's all right..
Maybe too much work at once for the gpu.
You could try to change some of these parameters,
I would start to try -w 256
-s form 100,000 to 100,000,000 default 15,000,000
-w from 32 to 32000 default 2048
-a from 1,000,000 to 100,000,000 default 30,000,000
-z from 2 to 30 default 10