A mobile chip on a desktop is unusual are you sure thats what you have?
That aside have you set the FSB frequency? Some boards you set it through the BIOS and some you set it via the jumpers.
Actually thinking about it, if its a mobile chip its possible this is correct, mobile chips in many cases shut down some of their features when not required as a power saving option to save battery power. And only burst into full speed when they detect this is actually necessary ie when you put them under load. This is one of the reasons they are more expensive that the desktop equivalent.