Thatll probably be intel speed step at work, this is a power saving feature which drops the multiplier from 8 to 6 when the system is at idle, this can be disabled in the bios under cpu configuration iirc, listed as c1e/eist, 8x400 fsb gives you a clockspeed of 3.2ghz, nice jump from stock, test theese settings with prime95 small fft, also what cooler are you using, as the stock intel cooler isnt great. Keep an eye on your temps with coretemp, ideally you dont wanna go over 70-75c.