Check the ignition wiring to the ignition box. The solder connection of one of my lead wires failed from vibration to the ignition box. Engine ran rough and at times would just quit. I chased my tail for weeks and two DS's until I removed the ignition box and found the problem. The wire just slipped out from the connector when I removed the box. Once repaired the engine ran perfect. May not be your problem but worth checking out. I also thought I had a bad ignition box, but it was only the wiring connection.