Batteries on cpu board, or a cold/cracked solider joint on the battery holder itself.
Anound 30 years of vibration/ heating and cooling of circuit boards = lots of wierd problems popping up. Hopefully the batteries didn't start to leak.
If you changed the batteries and the problem is still there, remove the cpu board from the backbox and check those solider joints around the battery box. It can really only be one or the other.