I put a new Facet pump in place of the old one that had fluctuating fuel pressure.
Also replaced the fuel line to the pump.
On test flight last Sunday again had low fuel pressure, as low as 1 psi, so I cut the flight short and headed home.

This morning I put a vent tube in the fuel cap pointing into the wind, to eliminate bad vent lines as a cause.
Pressure to outer carbs normal (about 3.3 psi), dropped a bit on run-up.
During take-off I noticed fuel pressure dropping more and more, down to .5 then .1 psi, at which point the engine began to loose power. Fortunately I was ready for it and pulled back throttle to the two outer carbs and was able to continue climbing and continue to a "normal" landing.

It's nice to have redundant system and be overpowered for the airplane, but I really need to solve the low fuel pressure problem to my outer carbs. (Center carb is shared between rotors and has it's own pump and throttle).
Pressure is OK at idle and apparently drops as fuel flow increases.
This would indicate a restriction on the pump inlet side, right?
But why is the other pump apparently not affected?
If the outlet path were restricted the pressure would increase, right?
New hoses from common point to both pump inlets.
This is the path (that has been working for 100's of hours):
Fuselage tank -> Fuel selector valve -> Fuel flow sensor -> fuel filter -> drain point -> split to two fuel pumps.

I've replaced the fuel filter in the last 3 months and 25 hours (cheap "G3" automotive paper filter).

Any ideas?

Finn