Had the same problem on a mondeo - its the micro switch which operates the boot release (looks like a rubber strip with a couple of bumps on), mine seemed to be worse after the car had been washed so must have been letting in moisture.
A new switch is about £30 (ish) from a ford dealer and is easy to fit your self if you are a confidedent DIY mechanic - you need to remove the trim panels on the inside of the boot lid which are a bit stiff but come off eventually, just make sure you dont snap the locating pegs off or else the trim won't be as snug as it should be and you may end up with a rattle.
MYB, December 2009
