1983 Other Plymouth Models Car Studders when accelerating

I have a 1983 Plymouth Scamp with a 2.2L manual engine. Recently my car has been shuddering or acting like its is losing power when I am accelerating. It usually happens when going through 3rd or 4th gears but not always. When I back off the gas it returns to normal but when accelerating again it dose the same thing. Usually if I turn off the car and restart it it works better for a while. I have changes all of the wires and plugs, the fuel filter, and the fuel pump. I recently noticed that the air hose going from the exhaust manifold to the air filter was off and put that back on but it didnt seem to make a difference. I don't have the money to have someone explore for what the problem might be. Any suggestions as to what it might be?

Update. I have the catalyc converter replaced and the car is still doing the same thing. When I was checking out ir the O2 sensor might be messing up I found that it had been removed and the place where it goes has been plugged. Could this be causing the problem?