2000 Ford Escort misfiring

Engine Performance problem
2000 Ford Escort 4 cyl Front Wheel Drive Automatic

I've been helping my son in law repair his 2000 escort zx2. It has a bad misfire at idle, when driving it will barely go faster than 20-25 mph. We replaced the plugs, wires and ignition pack with no positive results. I've looked online and see this could be a common problem with these cars, but I've never seen a solution posted on the various message boards. I really don't want to try the trial and error method as it can be expensive with no positive results, and we aren't rich. Please advise.