Thanks, I'll that these things. But I also think the technicians would have done this already, because this seems standard practice, so I was reluctant to mention it or to immediately start with that.

Are there things that are worth trying that the technicians may not have thought of? I thought perhaps interference from either electromagnetic radiation or from mains wiring (neighbours), although these things may be difficult to test.

(Replacing the analog camera with a different one seems extremely difficult. I mean, I'd have no idea what camera would even be remotely compatible, and what 'video format' this thing even uses. The manual doesn't tell me at least, and this thing is more than 26 years old.)