360 dies at idle, but not when in gear ??? Huh?

Hoping for some direction....have a stock 360 with a carter carb. The car has run fine up until today. It starts right up, goes into gear and will run like that all day. Put it in park and it stumbles and dies within 15 seconds. Fires right back up. I can understand if it stalled when you put in gear but this is opposite....stalls when you go to idle....any suggestions? Seems like a fuel issue but can't be positive