1973 Ford Ranchero repetitive distributor condenser failures

The problem started around 2 years ago. The truck is rarely driven, about 500 miles a year. (Vintage collectible truck) Kept in garage. I drove the truck for over 15 years straight before without this problem. Now the problem is severe, happening every hundred miles or so randomly. The engine runs fine.
The problem is a fail of the distributor points capacitor. The engine suddenly begins to misfire and finally will hardly run and ultimately goes dead. Replace the condenser and all is back to normal, for another hundred miles. I am using "auto parts store" replacement capacitors. Am I missing something here? I'm certain that during the first 15 years I had several of these replacement capacitors and didn't have fails. (I didn't get the original equipment capacitor because a dealership is not near me) This problem is very serious, leaving me stranded more than once. Any help on it appreciated.

ron