Missing cylinder in 93 Lumina

My girlfriend's car, a 1993 Lumina Sedan 3.1 V6 is giving her some problems. It has a distinct miss. It idles rough, and the RPM needle shakes back and forth at idle. It will idle smoothly for a minute, then the RPMs will drop, the car will start to shake, then something will kick in, and the RPMs will go back up. It like it is in a cycle of almost dying, then over-compensating, then doing it all over again.

When driving, the miss usually only comes during acceleration, or when going uphill, but there is a definite loss of power at all speeds. Also, her gas mileage has gone way down.

She took it to the first mechanic and he for some reason replaced the Catalytic Converter, but this didnt solve anything. The second mechanic ran diagnostics and determined that it is "the left coil pack and/or the control module." Since he wanted about $400 to do the work, I decided to take a look at it myself.

I determined that there is spark coming out of the right and the center coil packs, but no spark out of the left coil pack. I tested for spark right at the coil pack, so I know its not bad spark plugs or wires. I took the left coil pack off and took it to AutoZone to have it tested. According to them, it is just fine.

Is it possible for the Control Module to only be bad on one coil connection? I am sure that there is spark coming out of four of the six prongs on the coil packs, but no spark from the left coil pack. It doesnt seem to make sense that the Control Module would only fail on one side. Also, the fact that the car still starts and runs, doesnt seem like a Control Module problem. Every other car I've owned with Control Module problems wouldnt start at all.

I'm not sure where to head with this problem. I really don't want her to have to take it to a third mechanic, so any timely feedback to this post will be greatly appreciated.

I have a 93 lumina it is a common problem for the coil pack to go bad. This will also cause your module to go out if you do not replace the bad pack. Pull the wire off the coil pack if no spark, the coil pack is bad. Replace it. Even though it tested fine.

Believe it or not a module can go bad on its on and only cause one coil pack to not work......I have heard of this before so when I wasn't getting spark out of one coil pack I took the module in along with all the coils......The coils all tested fine and after the stupid guy figured out how to hook up my module to the test it, it tested bad.......I got a new one....about 110 bucks....It has worked fine for about 4000 miles now......