It could be one of two things:
(1) the odometer chip is on its way out since it's 30 years old and only had a design life of 10-15 years or
(2) the driver circuit for the odometer has a component or two that burned out.
The first one you can check easily by replacing the mileage chip in the odometer, but the second will need to be repaired like carlos said. If it's only the chip that's faulty,
Cehbra may be able to program a new one (since a used replacement will not show the correct mileage and the part is no longer available at a dealership).