Could you explain what this means, what it would cause, and how I can
fix it?

Why would it happen only when connected to a video source?

The same exact wires plugged into my other TV set works 100%, so they
are not grounded. But if it were grounded internally, it shouldn't
matter if a video source is plugged in or not. Right?

Homer J Simpson wrote:
"Jacob Aziza" wrote in message
ups.com...

I have found that this only happens when the antenna is plugged in
(actually the coax line is from the ReplayTV, which in turn is plugged
into a 12v amplified roof antenna via coax)

Sounds like you are grounding the set out - this is very bad and possibly
dangerous.