Loss of Power, backfiring - 1991 90 20vq sedan

[Brendan]

Ok, I made it a few years without wasting bandwidth, so here goes:

--- *Only* when in gear in driving ---
No matter what gear (but feels different in each gear), car will hit an odd 
powerloss between 3400/3600 RPMs if accelerated slowly through that band. If 
you rocket through, there is nearly no powerloss. This powerloss keeps going 
unless you downshift and/or somehow accelerate quickly over 4200 RPMs or so 
at which point, you feel the car drop out of wimpy and back into Hefty. If 
you stay driving in what I think of as "power loss mode", the car will 
backfire eventually.

Been happening for years, but I have adjusted my manner of driving to 
compensate, namely, I just speed all of the time.

New parts since problem began:
timing belt, water pump, tensioner
plugs, cap, rotor, wires, air fuel filters
wheels, tranny/diff mounts
entire suspension
exhaust, o2 sensor, cat
shifter
MAF was gently washed with alcohol until clean, but not replaced

Ideas? My mechanic and I are nearly out of them.