Engine idle

Hey guys,
So I have a 273 v8 in my 65 Dart and for some reason when the car is cold it runs and idles fine whether it is in park or in drive but once I drive it around for about 15 min and the engine warms it’s it wants to stall and idles really rough and bad when it’s it drive but once I put it in park it idles fine. Also I notice when driving it the voltage drops down on the voltage meter. And ideas?
-Anthony