[Mart]

Sometimes the rotor arm gets weak on the helmet distributors and allows the spark to short to the shaft the rotor sits on. A new rotor might help or pit some heat shrink on the shaft. This, like many rotor/cap problems normally shows when hot and "heals" when cold.
Does your car drive ok from cold and only have problems once warmed up? I have had similar problems with rotor and cap on crab type distributors too.

Mart.