Have a 85 mustang 5.0 CFI. One day I drove the car to get gas and it drove fine. Went back home and parked it. Went to leave within 30min and it wouldn't start. It was the MAP sensor so I replaced it. It drove fine for a day. Now it has a ****y idle and won't stay on. You have to keep the key in the run position to try to keep the engine on. I've checked a handful of things but not sure what to check.

 

I can't remember for sure but if there is a vacuum hose going to the map sensor then check it and make sure it is still hooked up at both ends and has no holes in it.
Good luck!

 

It was the ignition pickup in the distributor