After the last two days of testing (about 5 flights), I think I've learned a few things.

1) Line from pump to tank should be continously sloping up or down. Any "U"s in the line is likely to trap air, some of which is likely to be carried into the pump.

2) The more restrictions in the line to the pump, the better the tank vent has to be. Ram air pressure helps.

In tail-down attitude my fuel selector valve forms a bit of an airlock. Need to try another valve. I hope that Home Depot is open tomorrow.
Flow sensor inlet was lower than outlet. Handled by rotating it 180 dregrees.
With a straight clear line from tank sloping down to the flow sensor, sloping down to the filter, I saw an air pocket forming at the tank end, after reducing throttle from full power! Either tank pickup/strainer is too restrictive or vent insufficient. I put a vent on top of the fuel cap pointed into the wind, and I didn't see that airpocket forming again.

All in all it looks like my problem was a collection of issues.

Finn