Overclocked my 4670K to 4.40GHz @ 1.25v (uncore 4.20GHz @ 1.15v). Made serveral stress tests with AIDA64, all about 12h (CPU, FPU, cache, all together). No problems at all. But it bluescreens/freezes at prime95 after about 30min. Max. temp is 81C with prime and Aida's FPU stress.
But I take it thats no a huge deal, since pirme isn't a validated stress test and I've had no problems what so ever while gameing, which is what I'll mostly be doing (maby the occasional video rendering here and there).
You really shouldn't use Prime95 anymore. It is an outdated test and on the newer Haswell CPUs it causes them to begin throttling immediately and causes other instability and even damage in some cases. You really don't want to use Prime95. Stick with AIDA64.
yeah prime95 is indeed not the best tool to test anymore, however, your cpu gets probably to hot and starts to throttle, like all haswell cpu´s do, cause theire intergrated vrm module. if it works fine on aida then i would not botter to much about it.
But honnestly, what is the real world performance diffrence between your 4.2 OC and your 4.4GHZ.. basicly nothing... so i would personaly let it stick to 4.2ghz stable seems like a sweet spot for your cpu. you probably did not won the silicon lottery with your hasswell cpu.