I don't know if anyone had seen this, but the original problem sounded VERY similar (can't get 2500K past 4.1GHz).
Turned out it was a bug in the BIOS.
[I can't post links... google "Can anyone help me get my 2500K past 4.1Ghz", first page of the post was edited with details, so you may have to go to it if google dumps you onto page 17 or something]
May help. May not.
I registered to post it because I feel your pain.