It's a power saving setting. If it's not doing anything intensive, it dials back the power. If it needs it, it'll go back to 2.4. Don't worry about it. It only needs turning off if you start overclocking the CPU.
Just gave my Q6600 to my wife two nights ago. Great processor. DeeTee_uk is right. That's the power saving feature built into many Intel processors these days. It slows itself down when not being actively used to conserve on heat and energy use.
Only time you need to worry about it is if you want to overclock the processor. Other than that, it's actually a good feature.