Ok, more info here.

To recap -- an 85 westy with a rebuilt motor with about 500 miles on
the rebuild.  Immediately post rebuild, it would idles somewhat
roughly, smelled like it was running rich, poor fuel economy.
Drove it around for about 500 miles, then I took it to a different
mechanic who did oil change and adjusted the richness, after which
point it smells less rich, gets better freeway mileage, but idles
very poorly including intermittent loud "pop" noises as if one of the
cylinders is "frontfiring" up the intake pathway. Also has trouble
keeping the idle speed up when in gear (auto tranny).

Most of the FI system is new/rebuilt:  Distributor, wires, cap &
rotor, AFM, vacuum hoses, idle stabilizer, and even a new (used) ECU.

Today, I disconnected the O2 sensor, and like magic everything
improves dramatically -- absolutely no more "pop" / frontfire noise,
idle is much smoother.  It does smell a bit rich at idle.  Power on
the freeway seems about the same.

My dilemma -- do I assume this is all the result of a flaky O2
sensor, and just fix it myself or have a local shop do it, or is it
still possible something is seriously wrong with the engine (sticking/
broken/misadjusted intake valve) and take it back to the shop that
did the rebuild?

To add to the decision complexity, it's about an hour drive each way
to the mechanic that did the rebuild, and I'm moving permanently out
of the area in a week...