hi Ihave a 1991 gmc s15 jimmy 2 door 4.3 4x4 idle problem at stop light

What is your question?

up and down the idle seems after engine warms up unsure if related but but then stalls also going 50 mph 1250 rpms take foot of gas pedal takes forever to drop to 1000 then never wants to idle down to 750 can't figure out engine was replaced from a monster auto in michigan at 113 k thanks to me breaking the stupid oil filter assembly tune ups fuel filter so much replaced to no luck please sos Lucky