I'm with Bushy. I'd check Fire first. Mine was doing the exact same thing. When it ran it would run great and then start banging out on one. One + wire feeds both of the coils so you know you have power right before the power splits to right and left coil. Pull the tank. ( Check it for water while your at it) Double check the connections, plug-ins at both coils and wire to points. Mine were loose in there connections. As a stop gap measure, I cleaned and crimped them just a bit then zip tied the bundle to the frame to keep them from flopping. (It's on my list to replace the plugs with high quality model aircraft plugs)
thu