My HP pavillion ze4805us system speed keeps changing from 509mhz to 2.12 ghz, why does it do this?

Most laptops vary teh speed of the porcessor as needed, as a power saving measure. Less speed uses less power.

When the laptop is idle it drops the speed and increases as more is needed.

You may need to update the bios. It may need to be flashed. This will need to be performed by someone who has extensive computer knowledge. The primary function of the bios in a computer is to load the operating system. It determines what type of hardware you computer has. A current bios is stored in a bios chip located on the motherboard of the computer. It is also known as firmware. It can be downloaded on the HP web site and the flashing performed by yourself but be careful to read how it is performed included in the documentation. Otherwise you could cause your computer not to start at all. Be careful if you are undertaking this task.