Thanks, everyone for your help. But we were all wrong.

What appeared as fuel supply symptoms were not relieved by replacing the filter, FPR, or both pumps (or by checking lines with vacuum pump/gauge). (goodbye $500, a couple of Saturdays, and a lot of time reading through Steve Ringlee's FAQs--time well spent, by the way)--The problem turned out to be a (very) loose FI ground connection.

When reaching around blindly to loosen the distributor mounting bolts (checking Hall's sensor connections), my eyes happened to be looking at the intake when my arm brushed the ground strap running from the intake to the firewall--the wires moved at the fuel rail mounting bolt, which also serves as the main ground connection for the injectors. Apparently I cross-threaded the bolt when I put seal kits on the injectors about a year ago. It took months to work itself out about 1/2", loose enough that the wires bounced around with engine vibrations.

I just switched the two mounting bolts (good bolt to bad hole and vice versa) and tightened them down well. All stumbling/stalling/AND poor mpg went away.

The main lesson here is to NOT skip over the first instruction in all troubleshooting guides: 1)First, check all connections for tightness and evidence of corrosion.