No, what I meant is cap-coupling between supply GND and frame (earth). I've often had good results by finding the connection between frame and circuit GND and replacing it with a 1 microfarad nonpolarized cap.

I've never considered why this might be, but, historically, I found it on a multiboard system (in the late -'70's) when I replaced a board that was cap-coupled to frame with one that was directly connected, and found that the system had all sorts of peculiar behaviors. Replacing the connection with a cap made those peculiar behaviors go away.

RE