Tony,
Because you have a 12 volt system, you need some method to reduce running voltage to the points. Your choices would be a ballast resistor in the primary lead wire, or a internally resisted coil.
Your problem sounds more like a fuel issue to me. Electrical gremlins can come and go, but fuel problems are a more likely suspect.
Is your fuel tank vented and clean. The engine doesn't take well to water, so a charge of a gas additive might be worth the dough. Next time it happens, try choking the carburetor and see if that makes a difference.
Stuart
