Thank you all for your comments and interest!
   Here's the saga: I put in an Audi 3A code engine (2 liter 8 valve)
mounted on diesel parts (@ 50 degrees), and managed by a Bosch Jetronic LH
2.4 FI system taken off a 1990 Saab. The FI system does have an air mass
meter and basic diagnostics and is a pulsed injection system. I tried this
based on experiments, research, and musings done by Frank Grunthaner for
his turbocharged 3A engine (see archives).
   It works well, other than the RPM fluctuation at idle and throughout
the speed range. Scott, I have 3 other used air mass meters and have
swapped them in and out with no effect. Do air mass meters slowly wear
out? I hadn't heard that before. The diagnostics show it working.

   Have I4 swaps generally had problems with rpm fluctuation?

    The return hose I pulled off was the hose just downstream of the fuel
rail (only one on my engine), just after the pressure regulator.

    The new aftermarket tank has a steel line (unlike the vanagon plastic
tubing) attached to the bottom of the tank and the tank is half full so I
don't see how air could be being drawn into the submerged line even if it
was broken. The hose from tank to pump is new as are the hose clamps, and
doesn't leak gas at any time. And when the old original tank was still in
place I could see air bubbles swirling in the old small plastic pre-
filter. I replaced the tank and eliminated the filter because I thought I
had a restriction. And bought the aftermarket tank partly because the
outlet pipe was 12mm, not 7mm like the original, so I hoped for more flow.
    I'll try using a gas can before the pump and see what happens. I have
no spare fuel pump, though.
    Mostly , I just hate being stumped.

              Any other suggestions?

                            many thanks, Sam