Mitsubishi Galant problem

I have a 99 Mitsubishi, 6 cyl which has a weird problem. The spark plugs and wires were replaced, oil was recently changed and the mileage is 56,000 or close. It drives ok for a while but then it has a ‘washboard’ motion which seems to even itself out. I hope I have given enough info to be able to diagnose the situation. Very frustrating.

Not even close to enough info. Washboard motion in what? How? When? Is the engine lugging, or is the car shaking around independent of the engine?

Alice, did they use factory wires or aftermarket? Aftermarket wires are notorious for causing ignition issues such as stumbling and loss of power.