Likely nothing wrong with the ignition system itself. Have you checked the battery condition? A 4.8v nicad of 800 to 1,000 ma capacity is plenty good enough to handle the ignition all day. better in performance than most nimh batteries.

I'm not familiar with the plug you're using, but if .018 works all the time why not gap it and leave it there? Plug fouling is easy to discern. Pull the plug and look at it. A wide gap requires more energy to bridge. This is an area I disagree with RC Ign. Although an ignition will function with wider gaps, they generally will not do so efficiently. The spark coils are forced to work harder and the energy requirements are much higher. The delivered spark will have a higher energy level but the capacitor may not have enough time to fully charge during the charge/discharge cycle. Higher rpm running makes the situation worse.