How can I fix the central locking for my 2001 Mondeo Estate?

The remote key is broken mor my 01 Mondeo so I have to put the key into the lock. Just recently the locking system seems to have a mind of it's own, when its cold or damp the car will not unlock at all, there is resistance to the key but no click. When the drivers door does unlock it takes several minutes until I hear the other doors unlock.