I was having the exact same problem with a HIS 6850 I just picked up this week.  Took me a day of researching and cruising the forums to figure it out.  Turns out it was a problem with AMD SDK 2.8 and even though I was uninstalling it and trying other versions, until I found a post that said I needed to manually remove the files amdocl.dll and OpenCL.dll from the c:\windows\system32 folder before installing another version of AMD SDK.  Once i did that then reinstalled version 2.5 all was fixed and shares were being accepted at ~210 Mh/s without OC.