It has been doing this ever since I built my PC (which was a few days ago) and everything was factory defaults then. I even removed the CMOS battery and its still doing the same thing
Your CPU will by default clock down when there is no task that needs the CPU power. And crank back up to full speed when something demanding happens. To check if this is really happening, open a game or something, and then check the CPU speed. If its still happening post back here.