This one is odd
This with happen once and then nothing for 2 or 3 months.
91 full size bronco 302 5 speed
Turn key and start engine.
Starter stays engaged.
Turn key off.
Starter still cranking
Pull batt cable off battery
Take batt cable off on a few more times
Fine for a few months
Solenoid has been replaced 2 times
What else could it be?
Is there another relay?
I still vote for the soleniod. Let me guess, soleniods from Advance or some other auto parts store? We have been through probably twenty of those crappy things. Go to the dealership and get yourself a Motorcraft soleniod.
Maybe your starter switch is malfunctioning. If it does this again disconnect the I wire from the relay and check and see if it's hot with a test light. If it's the starter switch then it should stop cranking when you disconnect that wire too.