R100S occasional sputter and idle issues. Coil?

[Wobbly]

I have to agree with Kurt. Normally plugs will go 20K miles, but you can foul one in an instant with old or contaminated fuel. Or, if your commute has the engine chuffing along in stop-n-go several miles during your ride, you could have fouled a plug. Or, your spark plug wires can be old and leaking, robbing the plug of the ability to fire. Any of these can in turn burn out a coil.

[Roy Gavin]

You might also have intermittent high resistance in your ignition switch or elsewhere in the ignition circuit, not much works well without the correct volts.
You could try running a separate feed to the coil, perhaps through a relay operated by the stock feed, and see if this helps - not that common but if you are running out of ideas------------.