There are a few reasons for this, but in the absence of any obvious leaks, there is a spring in the shape of a cone that controls this and over time it weakens and shrinks, this causes the pump to 'hunt' as you describe.
This spring is in the end of the pump and fairly easy to get at and should be 22mm long, you can simply measure and then stretch it back to size, this cured mine and quite a few others!