I have an ATI Radeon 9800 Pro, with an Svideo output. I have this
output going to my TV with a standard Svideo cable, and there is
significant interference - diagonal lines that typically wave right to
left with equidistant spacing, but random velocities. I have tried
replacing the Svideo cable, and plugged the Svideo cable into another
output device, and the interference still remains.

I know this question will require clarification, but I don't know what
other information to provide. I'll answer clarification requests
promptly.

What is causing this interference, and how do I rectify it?

Thanks in advance!