Hi Tom.
The PSU the camera is normally fed from (together with three smaller cameras on a power splitter cable) is 5A, and that was more than enough when I totalled the power requirements up originally. But the fact it's one PSU powering four is what prompted me to test the affected camera on it's own PSU just to make sure and that test PSU was 1.5A.
But your cable comment might well be significant now I think about it, and have done some checking......
The first system I ever put in was one of those complete Swann DVD and multiple camera kits many years ago (yes, I know - I didn't know any better!). There is nothing left of that except two cables - one between this camera and my current DVR, and the other is for a small HikVision HiWatch camera which is in a position that is rarely trigged on an evening so I can't even find any footage from when that switches to see if it has the same problem.
Both these were difficult cable runs when I originally put them in, which is why they are still there.
The other three cameras I have are on high grade shotgun co-ax.
I've just checked and the Swann pre-made cables are very thin indeed and incorporate the power too. What I think I'll do is just rig up a decent test co-ax one evening and see if that fixes it, and if it does, I'll re-cable it.
Thanks!