What does stock voltage mean? Depending on the motherboard and the bios version, the AUTO setting may result in too much or not enough voltage when you are overclocking. Try running CPU-Z when stress testing to see how much voltage your CPU is getting when loaded.
Are you using LinX with the updated Intel Linpack libraries? The newer libraries use the AVX instructions like the recent versions of Prime95 do. Stasio works overtime maintaining his useful software list if you need to upgrade your version of LinX.
What memory speed and timings are you using. Slowing your memory down might allow your CPU to go higher.
At the end of the day, this probably isn't too important. The difference between 4.2 GHz and 4.0 GHz is only 5%. You might notice that difference during benchmarking but you are unlikely to ever notice a difference in gaming or during day to day use.
Intel designed Turbo Boost so when less cores are active, the remaining active cores could Turbo Boost up to a higher speed but most users on OCN that own K series CPUs disable that feature and just run the same multiplier whether 1, 2, 3 or 4 cores are active.Edited by unclewebb - 7/2/13 at 9:34pm