1998 saturn sl-1 idling poorly in gear

I have a 1998 Saturn, automatic trans.,SL-1, 1.9L, Vin #8 engine. The car has 271,000 miles on it and runs good as soon as you touch the accelerator. The problem is while you are idling at an intersection. When the car is first started in the morning, it runs good until the engine temperature raises to the normal operating range. It then begans to miss and chug, shaking the entire car anytime I am sitting still. The rpm guage will fluxuate from 1200 rpm down to 750 rpm, back upto 900 rpm then back to 800 rpm. I have replaced plugs, wires and had a diagnostics ran on it and have had the following messages show up:

1. P0340 (OEM brand OBDII). The pcm has determined that a malfunction exists in the electrical circuit for comshaft position sensor.Probable causes = cam shaft position sensor defective and or timing belt/chain out of time. I know that there is not actually cps on this motor.

2. P0304. Pcm has detirmined a misfire in cylinder #4.

When I remove plug wires individually off each plug is has no effect on the way the engine runs on any of the plugs. All of the plugs are firing ( Spark is firing at an extremely fast rate (almost a solid arc)with no variation in firing. Is this normal? Appears to be identical on all plugs.

How can I tell if the coils are bad ?

How can I tell if the DIS is bad?

Any other ideas?





Would sure appreciate anyones help on this animal.
I drive 200 miles a day to work and back and can't afford to get another vehicle.

Thanks

that particular engine does not have a cam pos sensor, it is part of the ignition module. I would replace that, both coils , plugs and wires. Also have you had the pcm coolant sensor replaced this part has been known to cause a variety of driveability problems on this engine. Hope this helps!