904 flare up issues

Same problem on both my 83 Dodge van (318/904) and my brother's 83 Dodge van (225/904). My 904 is the 1983 original lockup version, my brother's 904 is a 1973 non-lockup unit out of a Dart (there's the A-body link).

Symptoms:

Slow engagement into 1st, random flare up at 1-2 shift, flare-up at 2-3 shift, flare-up at 3-2 downshift or kickdown-2 shift.

Already adjusted the front band (the one accessible on the outside of the transmission case), going to be adjusting the rear band in a week or so (the one you have to pull the pan to do). Transmissions are full with ATF+4 and fresh filters.

What might cause these flare-up problems? How do I diagnose them? I have a shop manual and gauges to test line pressures, should I be using those?

While we are in the transmission pan, what tricks can we do (short of installing a shift kit) to firm up shift and reduce gear overlap? Remove accumulator spring? Adjust line pressure? ???

Transmissions are completely stock, no shift kits or other modifications have been done. Any tips will be greatly appreciated. I hate knowing nothing about how an automatic transmission works....