Sounds like your regulator got stuck in full return mode, just a big ol dump back to the tank. When the pressure slowly started to build, the regulator spring or whatever it uses was starting to neck down the return thus building pressure. I would look at the regulator for starters. Did you notice the pump frequency change during the "healing" process? It would be difficult to know whether the pump was the culprit or the regulator but if the pump was floundering while you had 20, that would be the pump. It should have been going pretty good at 20 if it were the regulator.