I have 1990 Transport 3.1, auto, that dies when I stop, after it gets warmed up! I have to put it in natural and then start it again. It starts right back up no problem,and otherwise runs great! When I'm stopped the rpms drop and then it dies? I have had the oil changed, changed the spark plugs and the wires, had all the fluids checked, and it has a brand new battery. I don't know anything about cars so any help would be greatly appreciated!! THANKS!!!!