popping during acceleration

I have a 88 5.0 mustang 5spd that during acceleration (3500-6000) and gears 3 thru 5 I will get popping even if not jamming through the gears. Thought I had water in the gas so I put water remover (x2) and car seemed to do better but it's still popping. Changed plugs, cap, rotor and fuel filter but no change. I have a long trip coming up and need it running good.