1991 lincoln mark vii lsc runs but not when let the key go back t

I have a 1991 lincoln mark vii lsc. i can start the car with the key but if i let the key go back to the run position the car will shut off. if i hold the key forward just shy of engaging the starter i can run the car, but have to hold the key, but nothing inside the car works when i do this(wipers, radio, command center....etc). every once in a while the car will run just fine , everything works properly. i have replaced the starter sileniod, starter, ignition lock(for the key). i am starting to think that it may be the computer in the car. i don't believe that it is the neutral safety switch, i can't start the car unless it is in park or neutral. can anybody help or have had simular problems like this that can point me in the right direction??????