The thing that is strange is that the pump goes off when you purge the system but then by the net morning it is on and if I read right it stays on. A small leak on the pressure side or debris in the back flow valve would cause it to cycle on but once the pressure came up, it would shut off and them recycle again. It is possible there is debris in the pressure switch preventing it from seeing the actual pressure in the system and thinking it was in demand mode. Purging would release pressure enough to momentarily clear the system as would any high water demand, however slow drop in pressure overnight might not. It could also just be a sticky pressure valve which would do the same thing. During your testing when it gets stuck on after not being used for a while try tapping the pump housing lightly to see if the sensor will activate.
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