Couple of off-the-wall datums:

1) Have seen postings on the RV-list of fuel pressure inexplicably dropping at altitude. Just an observation. The handling was "don't worry about it - that's normal".

2) Was told by an A&P that twins (higher altitude planes) usually have in-the-tank fuel pumps to eliminate pressure drop from pickup to the pump.

3) My own fight to eliminate air or vapor pockets in feed lines to pumps and simply giving up. I have 3/8" ID gavity-fed feed lines to pumps, continuous down-sloping. No valve, homemade Y-splitter that doesn't change the ID. Plenty of free-flow (30 GPH), but still see air/vapor pockets form when turning off one pump. Lines and pumps do not feel hot to the touch. Even tried RAM pressuring the tank (fuselage tank) with little improvement.

There is some factor (variable) that has not been nailed down here, and I suspect it has something to do which evaporation (change from fluid to vapor state) when  pressure is reduced by pump suction.

"Normally" it is not a problem, but under some wierd combination of circumstances is can become a problem.

There have been 100's of postings on this list on the subject, but I'm afraid that the only complete solution is "pump in tank" and none of us want to hear that.

Finn