easy fix.. its the Speedstep Technology dropping the multiplier when its not being used fully, if you want it constantly at 2.4ghz.. disable it in the BIOS, if you run 3dmark06 or something that uses all 4 Cores @ 100% Usage, it'll go up to 2.4ghz
This is due to Intel's Speedstep, which lowers the CPU multiplier when it is idle and raises it when it is on load.
To disable speedstep, turn of E1ST in the BIOS.