1990 Chevy Truck Lugging Engine

Engine Performance problem
1990 Chevy Truck V8 Four Wheel Drive 156000 miles

I have a 90 Chevy Ext. Cab 4X4, automatic. It seems to only run smooth when I have my "foot in the gas". It "chuggs" really bad when it kicks into OD when just cruising along 20 to 50 mph. I always have to gas it and make it shift down to get the rpm's up so it will stop chugging. It has an intank fuel pump, which I have replaced and emptied the tank thinking it was full of rust and debris. I have replaced the fuel filter numerous times before changing the fuel pump. Sometimes it would have junk in it sometimes it wouldn't. It acts like it is starving for fuel when it has to shift into the OD gear at low speeds and also at highway speeds. And would do this even right after changing the fuel filter.