I have had the same problem with my 2005 F350 since new. Ford dealer gave me the same story-everything was supposedly OK. I isolated the problem to using the cruise control-if I don't use it I have no problems. So when I use it I reset it every 15 minutes or so. Lousy, but it works.
I asked the Ford dealer to install a 7-pin bed plug and they refused-said it would cause problems-but if I wanted to do it I should use heavy low-resistance wire and solder the connections.
Somehow circuit resistance plays into the problem, but only when the cruise control is on.