Start with connecting a good old CRT in stead of the LCD to see what happens. It's an easy swap.
If that's OK I suspect the monitor itself. If that's wavy either, I suspect the videocard (or the driver, but that's a rather theoretical option, I think).
Help - my flat monitor screen is now "wavy" whenever I scroll up and down on a page. I don't know why this started or how to end it. Any suggestions?