Have you tried another S-Video patch-cord?
Some cheaper ones short across pins.
What output from DVD are you using?
If composite video 'Yellow' RCA socket, a cheapo cable can also create sync problems ie signal timing, causing patterns on screen.
The S-Video is actually 2 signals - one for Chroma C and the other for Luminosity Y. With S-Video to S-Video connections, this gives better quality than composite, as each line can handle separate voltages, levels and frequency for Chroma = pixel color and luminosity = pixel intensity.
Composite has to cram all this info onto one channel, so signal quality is lower than S-video.
A hack that videophiles use is to place a 470nF ceramic capacitor across the 2 signal pins, instead of the cheaper just bridging them together.
This apparently helps to re-sync the combined signal from composite.
A tester on ohms and steady hands can test whether your cable is cheap-shorted type, or better synced type.
Across C and Y pins:
high reading or open=small sync cap
If making own cable mod:
Pins are closely spaced, so easier way is to strip and dress cable near S-Video plug, solder wires to cap accordingly, join composite to cap & Gnd as shown, insulate.