I've heard of condensors shorting when they get hot. But I've never actually seen it happen. If it does, the spark will either completely disappear or become weak and feeble at the points, coil. & plugs.
More likely to be old plug wires, connectors, & dist. cap.
Test the coil by connecting the secondary to a spark plug and holding the plug against the frame while touching the coil primary connections to battery voltage.