Make sure you set the correct tvout connector type:
Option "TVOutFormat" "SVIDEO" or "COMPOSITE".
And perhaps the TV standard you chose isn't correct. (PAL-*, NTSC ..) The drivers default to NTSC. Check what TV standard your country uses. (if you have PAL most of the time your TV supports NTSC too but PAL is better ..)
Option "TVStandard" "NTSC", "PAL-B" ... Check the drivers for the standard your country uses.