The pump has a built in pressure sensor. When the pressure sensor senses the pumps cut-out pressure, which you stated is 40 PSI, then the pump stops running. With the switch "ON" the pump will sense a "cut-in" pressure, when the pressure within the system lowers, and the pump will start up again. I sounds as IF there may be a problem with the "cut-in/cut-out" switch and when you bump the pump housing you sort of wake the switch up and it starts running again. It could be a bad electrical connection internally within the pump also. Being as you have never taken one of these apart I would recommend that when you do remove the pump have a place you can sit down to take the pump apart, have something like an old towel to lay the pump on, there will be a little bit of water in the pump. Take the pump apart, lay the parts out so that you can reverse the way they are laid out and inspect the internals of the pump. You may find some loose electrical connection, and maybe some sediment within the internals of the pump. Which brings to mind--do you use a filter between the fresh water tank and pump?
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