I had an Intrepid with the 3.5l engine that did something similar. Drove me nuts for awhile!
Try accelerating it hard when it starts misfiring. Probably, it's going to give you a code and it might shut down, so be close to home. The code will be for "inadequate dwell for engine load."
I finally dug up some schematics and found a diode in the harness at the rear of the engine on the passenger side. Trace the harness back from the coil pack. The diode is shrink wrapped in the harness, and has 1 wire to it from the coil with 2 wires coming out the other side. I fixed mine by cutting the diode out and replacing it with a condenser (yep, think old school point ignition) from a 68 for pickup that we had. Worked like a champ. Just taped it up in the harness and put it all back together and drove the car another 2 or 3 years.
Wednesday, August 4th, 2010 AT 9:22 PM