LAMBERTech

I didnt see a trouble shooting section. I hope my question is ok here. I have 2 cameras run around 400ft on UTP cable with passive baluns. Im getting a wierd look on both cameras. It looks like clouds are passing over head and causing shadows but one camera is indoors. When I go to the head end and put a test monitor on the cable, the effect dissapears on both cameras. as soon as both cameras are hooked into dvr at the same time, it comes back. Has anyone seen this before?

Thank you rory. The problem turned out to be a ground loop, for anyone who runs into this in the future.