Sounds like you've got the bad solder joint problem. The fix is to pop
out the unit and retouch the solder joints on one particular connection
point. You can find a step-by-step guide on the following site:
www.fordf150.net
Yours may be an Expedition as opposed to an F150 but I would suspect
the same problem with the same style fix. I did mine on my 99 F150 in
just under a half hour. Good luck.

Cheers,
Lawrence