Hello again,
I was checking the ES250/2 again earlier this year and found that the problem was corrosion in the ignition switch. The switch has little steel pins which extend through the base of the switch between the brass switch rotor section and the base terminals and the pins had corroded causing intermittent resistance. The switch was a new one but the pins don’t seem to have been plated so the steel started to corrode almost immediately, and the contact surfaces at the ends of the pins are very small. I made stainless steel replacements and fitted them with no problems since. You need to separate the upper and lower halves of the switch and the pins are visible when you remove the rotor.
Cheers, MikeJ