My 89 4.2 carb. 5 spd. did the same thing at low speed and it was the Ignition Control Module. A lot of times a lack of power could be a clogged cat. The fuel pump could not be providing enough fuel to the carb. as you increase speed. I would replace it. Unfortunately, there could be a number of problems; carb, fuel pump, distributor, coil, Ignition Control Module, Cat. Have you been loosing anti-freeze. If not your plugs are probably o'k. I would start with the fuel pump and go from there.