Maybe corrosion between the distributor housing and the engine block, interfering with the ground? Try running a ground wire from one of the cap hold-down screws to a side cover bolt, or some other known good ground. Not likely, but possible.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!