central locking system?

hi. ihave a 51 ford focus ghia.the doors will not open by the key fob, useing the key on the drivers door sometimes opens all doors other times only the driver side.useing the key on the passenger door olny opens that one, same with locking them.other times all doors lovck when driving along the road,have tried the switch on and off 4 times,works now and again but not a permanant solution. help needed.