'Contrary' Tailgate Central Locking

ok, odd one for you... the CL on my tailgate sometimes decides to do the opposite of the rest of the doors  ie, Lock up the bus and the tailgate unlocks, unlock the bus and the tailgate locks... sometimes when the tailgate is locked, the key won't open it and vice versa... you get the idea. Is this, as I think, a case of a worn cam/gear wheel inside the motor? Has anybody else come across this?
Any help appreciated

thanks
c7

After checking as above, odd things can happen if there is an intermittent open circuit,check for and clean any "crud " from the contacts, making sure you have continuity in the right places at the time of switching, also worth having a look at the earh connection that goes between the tailgate and top left corner of the body (on LHD), another cause of it "having a mind of its own ", is if the pop rivets have worked loose that hold the motor to the tailgate, good luck searching!!