why does my boiler pressure fluctuate regurlarly?

The pressure on my Vokera Compact bolier fluctuates from being either too high or too low. In most cases it is too high so I take out water from my radiators until correct level is reached, then for some reason the pressure works it's way up to being too high again. Why is this? I have taken the water from the radiators whilst the heating is both off and on.