I have never overclocked or built, so just take this for reference. Other people can comment on it as needed. These processors overclock differently than usual, I'm told.
Taken from an article in a magazine:
"Overclocking is still possible via the CPU multiplier, but only if you buy a K-series CPU. If you have a sufficiently powerful cooler, you can set non-k processors to Turbo Boost harder than usual, but if you want the most performance for your money, a Core-i5-2500k is the best CPU at the moment. We've compiled a step-by-step guide to overclocking the CPU from 3.3GHz to 4.5GHz, a safe everyday level, in an ASUS P8P67 motherboard.
1. Enter the EFI by pressing delete as the PC is booting.
2. Enter the advanced menu by clicking the button in the top-right corner.
3. Set the CPU multiplier to 45x
4. Enable Extreme Load-Line calibration.
5. Set the CPU voltage to 1.39V
6. Set the CPU PLL to 1.9V
7. Set the VCCSA to 1.1V
8. Set the VCCIO to 1.10625V
9. Hit F10 to save and restart."
I have no idea whether or not this is useful or I'm wasting your time, but I thought I'd post it all the same. For all I know you're probably doing things right and it's a hardware failure of some sort. Apologies in advance.