I seemed to have had a problem with Intel's speed step lowering the multiplier on my cpu too much and in trying to change it I broke my mobo. Now, with my new mobo, my cpu has it's full x8 multiplier but it's throttling because it's running at 75c. From the research I've done this is way too high for a non-OC'd E6400 with the stock cooling, I should be more like 45-50c.
BTW. Temps are from Core Temp.
Intel C2D E6400 2.13Ghz Bus 266 Mhz FSB 1066 Mhz
More aboutidle halp
How long have you had that CPU? I'd suggest you add better cooling but that non-OC overheating means that there could be something wrong with the chip. Unless you're based in a warm and humid country.