Welcome Thomas!
You need to get some brake cleaner or similar solvent and clean the top of the coil, especially the HV terminal. Your problem might just be due to carbon tracking across dirt. Also, examine the top of the coil for minute cracks or other signs of deterioration. If you find cracks, you may need to replace the coil. These coils should also have a rubber insulating boot covering the top, keeping it dry and clean. See if you can find one for yours.
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Doug
'81 SC Coupe (aka: "Blue Bomber")
Canada West Region PCA
members.home.net/zielke/911SC.htm