No tach bounce still sounds like a reference sensor issue. Did you remove the sensor bracket? If so the gap may not be correct. If you didn't remove the bracket, I would double check the connections and make sure you have the connections correct - ie ref to ref and speed to speed. If they are reversed, no signal and no start.
You can also ohm them out per Clarks garage but that is just a sensor check to my knowledge.
If you are the original owner, Congrats. Highly unusual and a sign of great taste
I know you'll get is sorted out. I know the ign sw causes problems and it MAY be your prob. but I'm pretty sure it won't cause a lack of tach bounce. Keep investigating before throwing new parts at it!
Bruce
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Bruce P.
2011 Cayman Base, LSD, Sport Chrono.
1988 951 totaled and missed
"You're the best I ever had" --JJ Cale