Strange problem with HT 40

[hell_pk]

Hello,
we have heitachi Seiki NK 40(also called HT 40) machine.Sometimes its "offset" button on display stops working.after couple of hours it starts working again.what might be the problem?

[hell_pk]

by the way system is 10T.We have checked the entire crt system including buttons,circuitry,pcb traces for continuity,replaced cards with another Ht 40 machine but nothing helped.problem might be in NC in back cabinet.
Moreover machine has an optical interface(a 2 wire connector) going from front CRT module (which contains offset button) to back cabinet NC motherboard.How do we chect this optical interface or cable.can we use regular continuity using multimeter.
any ideas someone?