Could be a bad Ground connection almost anywhere. The old TTL. DTL, logic was very sensitive to that with the gates.
Also a radio transmission very close by does strange things. Anything new installed in Machine Room?
The earthing is a good point and might be the first thing to try. When ever this is a possible fault I just place clip leads to earth at a few places in the shaft to check the effect. It might be getting a supply because of a floating neutral.
