Ground problem, I'm a Mopar nut but Chrysler has had a miserable time over the years with grounds on their lighting systems{DRL's especially}. My father's 1990 D250 does this well, we spent an entire weekend tracing the wiring to no avail. Check the sockets, chances are the wires are loose at the socket. If so, replace the sockets, if it's not there, don't spend too much time trying to track it down unless you 1. have a factory service manual & 2. know a good psychologist already, because chances are you'll go nuts long before you find it.
