You could be at borderline stability,and exceeding some temperature starts to make it unstable, that would explain why your oc is not stable with prime and BC2.
Youre on water tho, and the guy was just saying that not every C3 overclocks STABLE to 4ghz on AIR.
As to my stability its rock solid at 3.55 and 3d stable at 3.8 ,but i dont like my games hangin from time to time ;-).

Did anyone with Thuban tried to change multi with k10stat above stock ? Theres a slight possibility it would work ;-)