I came across a blog on the net by a mechanic who is also a writer. I located a contact for him and posted my problem to him. He responded with the following:
I do have a couple suggestions. Although it was quite a while ago I did run into problems similar to yours. Something I found out through my testing is that the number 1/4 coil going bad can cause a code 18, as can misfiring ignition wires. I had a 1994 Pontiac Bonneville with 3.8L in over a decade ago that had been to three other shops for a code 18 and intermittent misfire. They had replaced nearly everything on the car. It was the 1/4 coil that was heating up, missing, and setting the code 18. From your post I would assume you replaced the ignition wires. Another item to check is the ground for the ICM. It gets corroded and can cause an intermittent 18 code.
Now I have had the ICM mounting bracket off the car and I cleaned the ground connector and where it is mounted to the engine. I also did a "beep" continuity test using an ohmmeter between the mounting bracket and the alternator casing and got a strong "beep" out of my meter. I have almost forgotten the Code 17; spark reference, that was set at one time that went away. Maybe it is the coil pack; what does everyone think? This is the first positive lead I have had for some time.