1991 Chevy dies in gear

I have a 1991 Chevy and every time it’s put in
gear it stalls out it will do it in all gears
including neutral in park it will run all day long
and sounds flawless I have changed fuel
pump, idle are control valve , tps, distributor ,
rotor, ignition module , plug wire , spark
plugs, fuel injectors, temp sensor, rebuilt
throttle body, o2 sensor, map sensor, fuel
filter , and probably forgetting some it used to
cut out a little when under heavy load but
stopped doing that it tries to stall under
heavy acceleration but picks back up after a
minute it backfires every once in a while
when I was doing the timing I could get it
running pretty good but as soon as I hooked
the computer back up it would start running
super bad again it’s not throwing any codes
and I can’t rap my head around it and can’t
throw parts at it any more anyone have any
ideas