I have an all original 79 w/ a 400 that spits and sputters when you turn off the ignition, other than that it runs great. Starts right up, idles good, good power. It seems to wont to keep running. If I leave it in reverse when I turn off the ignition it seems to peter out quicker. If I put it in park and turn it off, I can get out shut the door and walk away with the truck still fumblin and bumblin. Any ideas?