OC the CPU to 4.2ghz has worked. It rarely goes below 150 now and never below 140 in the quick tests I have done. Anyone know of a good OC guide for the i5 2500k? I have just used the auto OC feature on my ASUS mobo...but as I have read this can damage the CPU over time due to too high voltages.
I'm sorry that nobody else is really helping you on your issue at hand. I've never had a Sandy Bridge processor, but I would bet it's the same as my Ivy Bridge i5-3570K.
Return clock speeds to default. I would then use offset overclocking, because that was the easiest for me and even kept the sleep states enabled. I think this is the preffered way of overclocking, it just seems that not a lot of people have done it and most off the OC guys try to stay "old-school" with vcore and LLC and whatnot. What you will want to do is increase your multiplicator by one, boot to windows and do a quick check of its stable (Intel Burn Test) if it is go ahead increase one more step. Once it is unstable, add + voltage in your offset.
I'm running my i5-3570L @ 4,5 GHz with a + 0,100 offset voltage perfectly stable.
Check your temperatures though while doing stress testing. You will at some point either hit a voltage or a temperature wall. In my case it was actually both. To go above 4,5 GHz I would have to substantially add more voltage and hence the chip would get to warm for my taste. So look put for your sweet spot and leave at it that. Don't forget to use Prime 95 for a long time test of stability!