I so sympathize. I struggled through these kinds of things for the first three years I owned my car. It's finally settling down as the gremlins one by one are exited. Keep at it!
Great car. My grandfathers car, similar, had the same pattern. Only other time I have seen it is here now. Congrats on the car. Gotta love that Chrysler green. I had a 71 300 in that color combo.
Great car, I suspect an electrical issue. Could be firewall related/wiring harness. Had a similar problem with my 66 300, was the wiring harness. When it got hot enough I lost spark.