Try changing the condenser, assuming it's still got an original ignition system. There's also a remote possibility of a coil breaking down as it gets hot, but I can count the true coil failures I've seen in the past 50-something years on the fingers of both hands, with a few left over!
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!