popping during acceleration

88 mustang 5.0 5spd. The car had a popping type sound during acceleration mostly 3 through 5th gear whether it's hard or light acceleration and about 3k. I've changed the plugs, cap, rotor fuel filter and used some water remover. It's not as bad with all those changes but still doing it. I have a long trip coming up need to make sure it's running good.