It's either that or a grounding issue, or a broken wire somewhere, but I'd swap distributors or pickup coils because you've done about everything else.
What kind of slop in the distributor? Can you wiggle the reluctor sideways enough to cause a misfire situation?

I am currently fighting with a ford 400 engine that has a hard miss in the #3 hole, everything else is fine. I'm pretty sure it's either how I put the distributor in or else the pickup is somehow missing that particular tooth, maybe by a few thousandths of an inch.
It can be very irksome.

R.