[Richard]

Sounds like the pressure is leaking off or you are sucking in air when the pump starts.

I would start by checking all your incoming water connections to the HH, keeping in mind that the 12V connections are intertwined in the same loop.

I don’t remember if the HH has a priming port, but if it does make sure it is not leaking in air.

Since my 12V is much higher than my 120V pump, I isolate the 12V loop with the appropriate shut off valves.

I suspect the air in the initial flow of water is confounding the Microphor. I wouldn’t touch the Microphor until I had figured out why the intitial startup of the HH was sucking air.