Don't have any experience with these systems myself being a confirmed points man ( if it works don't f**k with it is my philosophy ) but that doesn't help you. If its an unstable backplate ( which is a good theory ) you could try temporarily glueing a third lug ( or just a suitable nut or something ) on with JB weld. If it works you could make it more permanent perhaps by drilling through from outside, screwing just below flush then filling and rub down / paint to make it look nice?
If it is the backplate I would imagine it could also be hard to start as it moves about and puts the timing out, is that the case? Have also heard that battery voltage can be critical on electronic systems so I would check yours and perhaps give it a charge
