I have a +/- '57 chevy 235 that started to misfire on #5 after a 200 mile trip. Plugs are clean, rotor and wires are fairly new and I'm getting a spark. I don't have compression tester, but I am not getting any blue smoke out the pipe.
I hate to ask, but how do you know that its #5? I assume that you pulled the #5 plug wire and the engine RPM didn't change?
There are a lot of obvious, and not so obvious reasons why this could happen. For instance, did you check you intake manifold for bird's nests?
What I would suggest is that you borrow a distributor cap from a fellow bolter, preferably one whose engine doesn't miss on #5, and see if it makes a difference.
