SLC 500 I/O module become unstable (spiky values)

Hello

I have a bunch of MFCs that are controlled via AB SLC 500 analog combination I/O modules (1746-NI4O). After some time they tend to become spiky and cant hold a certain value. I have maybe 20 cards with this problem. The machine was installed 1994 so it is not so surprising. But does anyone know or have any suggestions about how they usually break down and how to fix them.

Kind regards Calle