C-Max tailgate door lock - door unlocks itself!?

We have an 03 Ford Focus C-max. The central locking is working OK, however the tailgate has started to unlock itself at will. With the car unlocked and all doors closed the tailgate lock "cycles" of it's own free will, opening the tailgate! Once the tailgate has unlocked itself the lock continues to periodically "cycle". If the ignition is off and the car locked it fortunately seems to remain locked. The lock cycling problem only seems to happen when the car is "unlocked", with the problem only affecting the tailgate, all other doors are fine. Is there a micro switch or similar that may need adjusting - any help is greatly appreciated!
Merry Christmas!