1990 Chevy Astro idles fine but loses power when parking or

Engine Performance problem
1990 Chevy Astro 6 cyl All Wheel Drive Automatic? Miles

I have a 90 astro with a replaced engine. It idles fine but loses power when parking or slowing down; engine often stalls when pulling out of a parking space and it sometimes stalls when going very slow. When cruising a good speed, I don't notice it, but when slowing down and letting my foot off the gas, there seems to be slight shuttering and an eventual loss of power. Tried using fuel injector cleaners, cleaning out the throttle body, and going to a mechanic. It stopped for a while, but after taking it in to the garage recently, it started again. (I also had my tires rotated at another service station around the same time.) But before it began to stall again, I also noticed a slight popping sound or hitching sound when backing up slowly or after a sudden acceleration from a stopped position. As the noises seem to go away, the loss of power issues began to come on. Also, it appears that my internal oil leak seemed to have stopped or go down to a trickle all of a sudden on its own. How is all of this related if at all, and do you think I should have the idle air control valve replaced to try fix the stalling problem?