It's an old car, how did you check compression? Did you hold the throttle to the floor? What's the numbers? I added compression to the list of what could be wrong with your car because some people don't know how to check it properly, but I did see you say you did.. also beleive it or not in my 2005 comp g I had a misfire the pcm didn't even detect, I'm just ocd like that and it turned out to be the maf on my gtp .. and the series 1 and series 2 are well known for maf issues. So bad actually that after market ones come bad in the box time and time again..
Also, when the light comes on do you feel a misfire? If I can detect a miss the pcm can't then I would like to know if the car is actually misfiring? For it to set a check engine light it would have to be pretty noticable. And if it does misfire does it do it so bad the cel starts flashing? There are 2 stages of misfires.. small ones the pcm sets a code for and bad ones that actually get the light to start flashing..
I know these cars well so I can help.. if you have a 2 channel oscilloscope it would help in diagnosing a faulty icm... Also some part stores can check them. And the ground wires for the icm and coils are right on the icm bracket and could be corroded and crappy
