I have a P4 cedar mill 631 (holding me over for a few more weeks till q9450) and I have been getting pretty brave with it now that I know I'm going to get a new processor, but I seem to have hit a limit of exactly 3.99ghz. Why? I messed with the voltage of the cpu ( all the way to the max Intel said was safe 1.325V). Is it a ram problem maybe or have I just hit the limit of the cpu. It boots all the way to windows at 4.27ghz but instant bsod. I know its not a heat problem ( or I'm pretty sure its not a heat problem) because in the bios it says about 35C. Anything I can do to maybe get the speed above 3.99ghz. It works great at 3.99ghz, no problems at all and it helps so much its not even funny.
I hope the q9450 doesn't have the same problem (I'm sure it wont)

Other info:
FSB: 275-280
Mobo: Asus Maximus Formula
RAM: Gskill 1000mhz ( hope thats not the issue because it was such a good price)

You might want to change the settings of your ram to unlinked. This makes it so you don't raise the speed of your ram when you raise the speed of your processor. Also, you might have to increase the voltages of your ram.

You might also be reaching the limits of the processor. Even though its not overheating, it just might not be able to go higher.

Sometimes with processors, you'll get good ones, average ones, and bad ones. You can have two identical computers and one processor might be able to overclock a little bit further then the same exact thing.

I would guess the ram but need some screenshots from cpu-z to really tell