ACF7P is a not bad, average cooler. But I think you are running past its limits.
Back when my Q6600 was my primary system, I discovered that I could run at 3.0 GHz t stock voltage with the stock cooler. With an ACF7P, I bumped into the Q6600 thermal limits at around 3.4 GHz. A TRUE let me run at 3.6 GHz.
You need better cooling or you need to turn the OC down a little.