Prototype

Hi all, I've got a CCTV camera that is jittering, as in the picture jumps up and down by one line every couple of seconds seemily randomly. It keeps setting off the motion detection. Does it during the day or at night with the IR on. Any ideas what could be causing it ? The whole picture moves down and then up by one sometimes two lines?? I understand how a TV picture is made up of even and odd fields or interlaced but why does this cause it to shift lines randomly??? The cameras have a coax cable and are 12Volt. I have a mains camera which also does this. PAL composite video, some running from a 4A PSU with battery backup, others with seperate PSU's. Diginet software Any ideas ? See link for example :- sorry i cant post links yet Camera in this example is a sorry i cant post links yet a few of my other cameras also do it but are different makes.