Try reversing the S-video cable (end for end) and see whether now touching the cable where it connects the cable TV box causes the problem (suggesting a bad connection inside the S-video cable).
Or just try a different S-video cable.
In general, S-video jacks are more likely to develop loose connections just inside them compared with RCA jacks. A new cable won't help here. This worsens the problem with some TV sets that have just one S-video jack that has to be unplugged completely before the yellow jack can be used at all.
Does the problem happen on all channels?
Video hints:
http://members.aol.com/ajaynejr/video.htm