Sounds like a problem i had last year. After about 3 months of jetting and troubleshooting i swapped out the ignition coil, stator, pick up coil, feul lines, feul valve, and fixed a couple of nicked wires. Then i cleaned and synced my carbs and the problem vanished. Point being that it could be a few things in combination. My ign coil and stator tested fine with an ohm meter but then i megged then later they were both leaking.
david