1994 Lincoln Continental Hard to start, coughs and backfire

Engine Performance problem
1994 Lincoln Continental V8 Two Wheel Drive Automatic

Son just bought this car from a little old lady. Its very nice, but she told him it had to warm up awhile before driving. It has to crank awhile to start, but even after uit warms up, it coughs, sputters and backfirwes when you press on the accelerator, and is undrivable. Car has a 4.6 fuelinjected, with 150,000 miles on it. We installed new plugs, air and fuel filters.
please help, he is 21, recntly married, and will lose his job if he can't get to work