I realize this is an old post, but I had the same problem and found a solution which might benefit someone in the hunt. I have the P8P67 with an i7 2600k running Windows 7.
Solution summary: Make sure that 'Number of processors' check box is not ticked in msconfig/Boot(tab)/Advanced Options...(button).
I recently noticed my Turbo mode was not kicking in past 3400 even though bios settings were all initially configured correctly and working when I built the system (OC'd to 4400). Tried many many things, (bios updates, playing with the settings, etc. etc.) but it turned out to be a setting in msconfig that was causing me grief. Sometime after I had successfully overclocked, I had gone into msconfig, the 'Boot' tab, 'Advanced Options' button and I ticked the 'Number of processors' box and chosen 4. The i7 2600k does have 4 cores, but each core can have a virtual core making the OS think it effectively has a total of 8 cores. Somehow, by telling Windows 7 that there were only 4 cores, it was screwing with the Turbo Mode. Anyhow, removing the check box for the 'Number of processors' fixed my problem... i.e. Turbo Mode now kicks in correctly to my 4400 MHz OC when under load. Hope this helps you!
(Originally I found that turning off hyperthreading in the BIOS got the turbo mode working again. However, who wants a 2600k without hyperthreading?... so after some more investigating I found the msconfig solution.)