If it's mis-firing, possible culprits would be plugs, plug wires, air filter, fuel filter, fuel pump, numerous sensors,.....How many miles do you have? What transmission? What rpm's and what gear are you running when it 'chugs'?
95 Contour Zetec, ATX (wrecked, sitting in the barn)
95 Mystique Zetec, MTX
2000 Taurus SEL 3.0L Duratec
1994 Crown Victoria LX 4.6 V8
1993 Chevy Silverado 4.3 V6