Could just be a rich low-speed circuit or
junk plugging the Idle Air Bleeds, or any of the preceeding.
The combination of rough idle PLUS low voltage indicated,
VERY often points to a too-slow idle speed, as @Murray already said
But it only happens after driving the car for about 15 min that it starts to act this way but if it’s in park it runs fine. I guess I could adjust the idle up a bit and see if that helps
