I have a 1990 C4. Having problems with the ignition. Complete ignition failure under full load at about 4k revs and above. I have discovered one thing. If i unplug either one of the ignition modules, it runs perfectly. Without a glitch. I'm wondering also if the distributor drive belt may have jumped a tooth. I've checked the alignment and they are very slightly off of the factory marks.

Has anyone come across this before? Any ideas would be great.

 

I doubt the distributor belt has jumped a tooth. If you suspect a problem inspect the belt and replace it if in doubt.
I suspect you have already found your problem, a failed ignition coil.
Ciao,
Adrian
911C4

 

Cheers. I'll give it a try. Is it worth replacing the ignition amplifier aswell?