58, After turning switch to off and removing it you need to get out of the truck so it thinks your done for the day.
Seriously, To much advance in timing as mentioned along with a fast idle speed and maybe running to warm.
I've only known engines to want to do that because of those reasons. Back in the 70's when idle speeds were a little high they stuck on a idle stop solenoid which when the ign. tuned to off the idle speed was dropped even further to prevent run on.
This only points out that even with a properly running engine if idle speed is high chances of run on are good.
Dan
