Jess echoes my sentiments on mechanical diode converters. Been there done that. As for converter guys saying they found nothing been there done that more than once as well. not saying that's happening but seen it happen more than once. Just had a buddy wipe out the thrust in his S/ST car due to a converter(big name shop), but they told him all was well. Oddle enough came back with a billet fro Kinda like finding a problem with and MSD unit that wont start a car yet they find nothing and works like a champ when it comes back

Plenty of validity to the cut off switch deal, seen that happen a number of times. Especially on older switches or hard leaving cars. Charging system voltage issue in general on the same style car. We had an alternator issue on the dragster that was causing the car to shut off when the shifter was activated. Car was still deadly consistent but we could not figure it out. Til it was on a single and we heard it shut off. The alternator was working just not enough to cover up the draw of the solenoid. If we had a date logger would have been much easier to diagnose. New battery and alternator and it was good to go.

Like I say be interested to see what you ultimately find.