Tested all of the above. Took the heavy cable off and turned the key and got load clicks on both syliniods when key was turned. Both of them are good.
Its somewhere in the ignition. It has to be. The switch itself feels positive and true when turned in all positions. Feels tight. I think in going to replace it regardless and somehow get it re keyed to the original keys. Those are cheap enough

 

if you turn the key to start hold it there (a buddy) and hear the solenoid click on and test across the heavy terminals and get 12v that is good

If you release the key to run you should hear the solenoid click off and you should measure 0v across the heavy terminals

if both of the the above are true then take the heavy wire from the starter and touch it to its post on the solenoid. There is a 99% chance that it will be fine but make sure the car is in neutral and just swipe it across at first to check for sparks.

 

tried and tested this, all good as far as the solenoid goes. next step is the ignition switch, this is a cheap test. only problem is it is very hard to get your hand under the dash without taking out the instrument panel cluster out.

I am thinking it is sticking itself on and off somehow, still doesn't answer the half dead battery in the shed theory. but its worth a shot, as of right now everything is A ok and no issues, car starts fine and runs great. scared to go out again and have it not shut off however. needs to be fixed for me to feel good about it and confident to actually take out and drive. leaving the battery disconnected for obvious reasons while it sits in the garage

 

was searching online for some related issues of this and ran into a discussion we had here on this site years ago. this could be a culprit !

""I had this problem on my F100. If you pulled the "I" wire it would cut off. The solenoid wasn't my problem at that time. The stator in the alternator was bad and sometimes the diode bank would back feed through the solenoid to keep the ignition powered. ""

this could possibly provide light on the whole issue of it being 100% one day then the next without touching it not. as well as the not shutting off the one time as well.?

 

The I terminal provides coil power during cranking.

I suppose its possible I dont know enough about that system to verify if the quote is true...It does seem plausible.

 

In Ford Alternator systems, the "I" connection at the alternator is not connected electrically to the Stator at all. It is the "hot" connection of the Field windings which feeds them current in controlled amount, this providing "Voltage Regulation" of the alternator output. It's output to the battery is produced by the Stator windings, whose output is fed to the diode bank, thence to the battery and operating system of the vehicle.


In cars with only an "idiot" light, the "I" terminal is connected directly to the light, the other side of the light being B+ powered from the ign. switch. In cars having an ammeter, the "I" connection is completely unused. Most Mustangs use a voltmeter, uncalibrated, which tells whether alternator output is acceptably high or too low, basically subjective information, rather than objective, indicating actual voltage. Those systems utilize the "I" terminal to provide an idiot light in addition to the voltmeter. imp

 

Another update. Been messing around with it and checking to see if anything has changed, everything A ok as if it never happened. Ran a few more test on the sylenoid and everything checks out. Took it for a short spin today. Starts up and runs great, started and shut it off multiple times with no issues. Still nervous about evrything however, scared it won't shut off and then not be able to reconnect the battery! I plan on changing out the alternator and ignition switch just to be safe. You think this is overkill ?

 

When the engine is running and you put your Volt meter across the battery terminal what does it read? It should be above 13.5v

 

Bouncing between 13.8-14.5 at idle

 

that would indicate the alternator is working