I have been trying to figure this one out on my 2003 Mustang GT with an automatic transmission. It doesn't seem to be an airflow issue so let me explain what happens. I can start the car cold in park or neutral and let it idle forever, it never dies out. As soon as I drive the car down the road and come to a stop as soon as I put it in park or neutral it will shut off after it reaches the Idle Transition period of 2 seconds. I know this because I extended it to 12 seconds and it idled for 12 seconds in the same scenario and then shut off. Now I can restart it in park and it will idle just fine until I drive it again. I have replaced the transmission selector switch with no help. I can rev it up once I stop and as long as I am depressing the accelerator it will stay running As soon as I go back to idle it stalls. Again I can restart it in park right away and it will run until I drive it again and park. It will do this in park or neutral. The car runs excellent otherwise. Idle is great a/c on or off. Slows down in traffic fine and idles perfect again as long as it is in drive. I can shift to reverse once stopped and it will continue to run until I either go to neural or park. Has anyone ever had this issue. This just all of a sudden started happening. It wasn't after any part addition or replacement. I have also replaced the TPS, cleaned the TB (Stock TB), Cleaned the IAC valve, replaced the IAC valve, tested the EBR, disconnected the EGR valve, Cleaned the MAF, Replaced the seal under the TB to intake, Checked all vacuum connections, sprayed starting fluid around the TB, Intake, vacuum lines, etc. Cleaned and tightened all grounds under the hood. There are no codes at all. Nothing changes it. Has anyone had this issue? Need some guidance before I go crazy!