I  had the same issue till i started using http:whatever + this firmware seemed to help a lot i was getting lot of  HW error more at defaults then i  updated CGminer and the firmware it help a lot, to day lone on the closed cover S3 after the 7 hours at defaults I  hit 10 HW errors with no rejects but i all so used some of the above stuff  it helped on top of the new update.