Usually rain and missing means plug wires are bad.  Also check cap for cracks
and carbon tracks.  Could also be a problem with the electronic ignition
connector on the distributor.  Unplug it and check for corrosion, and that the
pins are making good contact when it is plugged in.
Ken Wilford
Van-Again
John 3:16
PS Don't overlook the grounding strap on the driver's side head as a source of
potential problems as well.