Your question has been answered. Reduce your overclock, or buy a better cooler. Those are your options.
Yeah. To tell the truth, I never bothered even trying the stock cooler with my own SB-K when I built it. With the NH-D14 the idle is near room ambient at stock clocks and volts. I see the average persistent core values these days at between 35 and 40C at my OC settings, and I'd get uncomfortable if I saw them climb to near 50.
I just hope folks aren't still using a fixed VCORE method to clock their Sandy Bridgers. It's totally unnecessary even for find the stable settings as you go through the exploratory OC process.
My 2600K idles around 1.0V, and the voltage drooped value is around 1.34 to 1.36V, with the unloaded turbo voltage as high as 1.38V.
Seems to me that the 2500K only differed for not having HT. I thought they could be pushed easily to 4.5 to 4.6Ghz, but not having one -- what do I know?