My truck will die when I come to a stop (Rpms) drop. What's wrong?

What is your question?

When I come to a complete stop it starts idol n awful and then dies. I have to put it in neutral and keep my foot on the gas and put down into drive to keep it going. It seems fine when I'm driving and the Rpms are high. Just replaced the starter, the alternator checked out fine.At first it jut died occasionally , but now it does it a lot. Don't know what it could be.