After a couple of months happy motoring, I finally have a problem again.
The idle hasn't been quite as smooth as it should be for a bit, which I put down to spark plugs. But after changing them last week, it remained, and today it started misfiring. By the time I got home it was getting really bad, I could barely touch the throttle pedal without at least one cylinder missing, and often more than one.
Checking out the ignition system, we found that there was a hole on the side of the center turret of the distributor cap:
I still have the old cap and spark plugs (only changed them as a precaution, not because of a problem), but that didn't fix the misfire. Seems the hole was only a symptom instead of the cause.
I'll check the coil next, but don't have a spare one on hand. If that doesn't fix it, what could be causing this?