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Did you upgrade your BIOS? If you did, this would explain it.
The 3rd generation Core processors do not support the same set of capabilities for overclocking that the 2nd generation Core processors did. If you upgrade the BIOS on an 6 Series Intel Desktop Board beyond the point where support for 3rd generation Core processors was added, you will lose those overclocking capabilities that are not (also) supported by 3rd generation Core processors.
Just so you know, once you upgrade the board's firmware to add support for 3rd generation Core processors, you cannot downgrade it back to a version that only supports 2nd generation Core processors; this is a one-way operation.
Thanks Scott, you should try to overclock someday, it's fun
For those who will read this post in the future: Yes, overclocking through the BIOS works properly.
The BIOS offers two overclocking options:
1. Automatic: CPU overclock up to 4.5 GHz and no DDR overclock.
2. Manual: You do as you please.
But it seems to me that the Extreme Tuning Utility doesn't work well when going above 4.5 GHz