I found on my 70 Challenger that the wiring from the ignition switch where it exits the steering column and via the 6 wire connector(plus any optional wires such as key warning) that plugs into dash harness connector is where there is opportunity for much mischief.

What happens is that the male pins from the dash harness can begin to fit loosely into their corresponding female receptibles,,,thereby causing the random electrical circuit errors that you appear to be experiencing.

I created a somewhat Bubba fix that has resolved it quite well for my problem(s) in this area. The only true/correct fix is to replace both the ignition switch and the under dash harness,,,so far I've only done the ignition switch part.

The description of my Bubba solution, though simple, requires a bunch of words to describe how. If anyone should ask, I will post what I have done that seems to be holding quite well.

Meanwhile, if you do experience a start failure, just reach down, grab hold of this connector while cranking. If engine fires off, you know where your problem lies.

Also with a test light or meter you can easily test out all these 6 wires(they all are critical to starting and running of your engine) for continuity while wiggling this 6 wire connector about.

You also might separate this connector and spray some contac cleaner about,,,this May or may not solve the problem if it proves out to be in this area.