There are posts here and there on various lists and such about the tunnelling issue that go back some years, which has led me to think that caps are most likely unrelated to the problem — but who knows until it's tried?

The problem appears to be restricted to the first B&W/Greyscale active matrix screens (the "glassy" ones). Not all of them are affected, as on many of these old machines, there is still no sign of it. Maybe a technical flaw in manufacturing/ different branded components in the various machines in question?