One other possibility- - - - -pull the cap off the distributor and inspect the inside with a bright flashlight and a magnifying glass if possible. If you see something that looks like a very faint pencil line between a couple of terminals, you've found the problem. It's called a "carbon track", caused by the spark following condensed moisture inside the cap. Once the spark jumps a few times using the moisture as a conductor, a permanent low-resistance path gets established and replacing the cap is the only way to solve the problem. It's worth a look-see before more in-depth diagnosis happens.
Jerry
