doesn't run properley at a steady speed on 1994 Ford Escort

3rd gear is the worst. At about 30 miles per hour it wants to chug and jerks. Accelerating is just fine. It doesn't hesitate. But at a constant speed it doesn't want to run properly. And it only gets 25 to maybe 30 miles per gallon. I changed air and fuel filter and all the spark plugs.

Asked by for the 1994 Ford Escort
I would check your ignition wires, cap and rotor for wear.