start with new plugs
Cross-threaded spark plug.
Could be a bad coil (each cylinder has its own) common problem
i tmakes the car judder a i and the engine misfires
P1399 is a pending code for random or multiple cylinder misfires.
Your 1996 Pontiac Grand Prix will experience cylinder misfires when one or several of the spark plug wires are bad. A bad spark plug can also cause the problem.
Could be a dead fuel injector.
Chek out the ignition coils, the wires and the spark plugs..
possible misfires caused by spark plug/coil wires
You have to find out what is causing random misfires in the engine. See related question below.
2001 ford f 150 misfire on acceleration
Plugs, wires, coil, distributor, injector, internal engine....
the injectors....