You may have what is called a hi joint in the electrical world which means you have a high resistance connection that is going wide open when hot, now you already know it is in the ignition circuit so that limits the possibly of the problem part
You've already done a bunch of trouble shooting so now is the time to focus and fix it, the intermittent problems are always the hardest to catch and fix when they go open and cool down and remake the connection
Now get to work and fix it
You can do this
Make sure you have a good quality VOM and go step by step looking for the voltage drop between components and connections Start at the battery posts and go into the circuits to the ignition parts on the engine one step at a time
Do this test with the motor not running and write down the readings at each connection ands then do it again with the motor running fast enough to make the alternator charge at or above 12,5 volts Keep at it until you find that dirty little connection
How old is your ignition switch and lock?
Good luck, please let Ius know your results and findings