I would suspect the cable used to connect the Voodoo to the monitor or some component in between. It's probably not properly shielded for high frequencies.
Do you have a monitor switch? It can create bad output, too. Try to connect the monitor directly to the Voodoo.
What resolution do you run on the Voodoo? Does it happen with low resolutions, too, for example 640x480 or 800x600?
You could try to reduce the vertical frequency in Picasso96Mode. 60Hz is usually enough for flat screen displays.
