White foggy moving bar with nvidia tv-out
 

Hi all,
When I use the tv out on my tv ( ;) ), image is perfect but a white translucid bar is slowly moving horizontaly ( 5 cm width) and I don't like that so I'm wondering what should I do ?
I'm on Debian woody with Xfree 4.2.1 and latest nvidia driver. I'm using the nvtv tool (http://sourceforge.net/projects/nv-tv-out/) which is very good, on 800x600 .
Because of the cable is around 8 meters long, I'm thinking about signal loss...
Any advice or any test ?
Thx in advance