You probably have a defective Pressure Switch, being this is the main component that controls the cut-in and cut-out pressure of the unit.

You can try adjusting the switch, first; most pressure switches are adjustable. If you click on the below link you'll see a video and a document that has instructions on how to do this for most brands of pressure switches:

www.mastertoolrepair.com/how-to-adjust-p...-switch-p-30582.html

If this doesn't work then I'd recommend replacing the switch. Most likely the diaphragm is damaged/corroded so it's not sensing the correct PSI.

Hope this helps!

- MTR