Nm, is this a serial data lift? Is the HC-CIO the electrical isolation for the lift computer, or just the button module? My thinking is, if it isn't serial data I would try to swap the individual floor level trailers to the these floors to see if the fault moves to another level. If it is serial data, then it should use this for communication from the car control pcb(car top isn't it?) to the controller. Maybe that cable is getting noise or induced voltages or maybe the car control pcb is faulty. The fact that you have already changed the HC-CIO pcb, tends to think this isn't the fault. From what I remember the buttons aren't individually coded so have you tried changing the button modules or wiring in case this makes a difference? Anyway food for thought.