If that rig sets outside exposed to the weather (rain) I've seen the igniton switch rust inside enough that the stray voltage would intermittently get on the kill wire to the magneto and instant death and no amount of cleaning is a permanent fix for such.
If you do not detect any stray voltages using the voltmeter, replace the ignition switch and you have to keep a heads up about ignition switches, one size does not fit all even though the terminals on the rear of the switch seem to match.
Installing a wrong ignition switch will cause lots of problems.