Svideo cables will do this if they short out. Try another cable, or fiddle with it a little and see if you can find where the short is...
Desktop: Athlon 64 3700, 1024M RAM, GF 6200 TC 256M, 2x 80G, 2x DVD-RW
Laptop: Sempron 2800, 512M RAM, Unichrome, 60G, DVD-RW
Macintosh:G4 1.33GHz, 512M RAM, Radeon 9200 32M, 40G, DVD/CDRW
They get me from 0 to 1...