Had the same problem running a 6870. After 10 mins, slow down to 400kH. Registered just to post this solution.

Reverted back to version v2012-02-19 and has been running over 30 minutes with no slow down.

Good luck. Hope developer finds bug and fixes it.

If this has helped, please consider donating bitcoin to 18fQqUEfGHzA5FFxnC5j6Hy7wxnDehvN38