[Taf]

I have seen damaged cable cause total loss of signal when both ends of the screen were connected to the plugs/sockets. Internal shorts usually due to them being trampled or bent hard, and especially with air dielectric types.

Come to think of it, this can also cause the screening to become part of the aerial, causing signal fluctuations as you describe.

Looking at the picture, I have to ask how does power get to the masthead amp? And are the unused outlets supplied with loads?