Each processor is slightly different. No processor is built exact to the other (I don't know why that is), so overclocking is something that is either hit or miss sometimes.
I don't overclock my rig anymore. When I first got it, I did, and I got the 2.4GHZ to 3.2GHZ and managed 3.4GHZ, but I just switched it back and have kept it like that... It is making me want to try again though...