I assume that you've used the shore water prior to this and don't have any leaks, so take a look outside and see if water is puking out of your shore water inlet. If not then maybe your pump is getting weak, but that seems unlikely if is seems to work well otherwise. If you have the standard-issue for that vintage belt-driven Par pump, there is a pressure switch on the pump body. It has two wires going to it, so it should be easy to find, just make sure that the wires are hooked up. Generally if the switch was NFG, then the pump would run up pressure till it started to hammer, and cycle a lot slower as it tried to continue pumping against the excessive pressure. That would definitely burn it out. If it just runs steady, but won't quit, and you have sufficient fresh water in the tank then you must be losing water somewhere. My bet would be on the shore water inlet. Hope this helps. P.