Geovision Suddenly Dropped Video

For many months all has been working well. Then recently (nothing was touched) ch4 stating lost video.

– rebooted the box, nothing

– swapped the BNC cable with another camera, all works for 2 seconds and video is lost.

So no matter what I do ch4 comes up for 2 seconds (when I switch the to a different channel the video) and then goes away. How do I trouble shoot this?

ps

Any other video I put on ch4 works. ONly that one cammera on any channel will only last for 2 seconds.