I did notice a bunch of black residue (iron I think) in the 1/4" adapter hole off the main line.
I forgot to mention when everything was thawed out and I opened the hose off the main line when the pump was powered off and the gauge got to 50 psi the pressure would drop to 0.
The black residue is gonna build up in the pressure switch
over time, and the hole in the switch is pretty small....
probably led to your problem.
The "...drop to zero..." situation will occur when the diaphragm
in the tank reaches bottom....pressure/water supply immediately
goes to zero with no pump running.