Unstable ignition timing :
At some stroboscope lamps the ignition point suddenly oscillates around 4-6°. In this case the lamp once reacts to the ignition spark starting, another time to the burning end of the spark. If possible readjust the sensitivity of the lamp.

Doesn't start :
You can check the function of the ignition by unscrewing the spark plugs, leaving them in the plugs, connect to ground and by slowly kicking you can see or hear the sparks.

To check the cable connecting and the supply voltage put the rotary switch to '0'. Now the spark plugs must fire continually.

Irregular engine cutouts:
If sometimes the engine suspends while driving for 2-3 seconds and keeps running thereafter normally. That means that the ignition has been reset. The cause for it can be a defective cap or a loose ignition cable in the coil or cap. But in most cases a bad contact in the operating voltage supply (starter lock, fuse holder etc.) causes tis effect. For a test you can put a cable directly from the ignition coils and the ignition box to the positive terminal of the battery. Also put a second cable from the negative terminal of the battery to the ignition box (secure ground connection). If the engine is running well now you can assume an error in the wiring harness.
With contact brakers such a bad contact is not noticeable, because a short break for a few milliseconds of the supply voltage does't matter, electronics in contrast are more sensitively.