my buick rendezvous door locks have begun locking and unlocking themselves while i'm driving. it seems to happen after driving for awhile - not just short trips. the locks begin switching slowly and then they get faster and faster. after doing this the other night the locks themselves stopped switching to open and closed, they remained locked and there was a clicking sound low/center of the dash. any ideas what could be causing this? thanks for any help you can provide. our dealer has no idea.