Oh... actually, wait a second. It could be ringing, but usually you'll see more than one "echo". It just occurred to me that I've seen a very similar display fault with a sync-on-green Mac display card, when I've had the separate sync lines (Vsync + Hsync) connected at the same time. If your adapter is a simple passive one (because you have a sync-on-green capable monitor) then the only lines that should be connected are R, G, and B. Don't connect anything to Hsync or Vsync. If you have green connected to green as well as to Hsync and Vsync, this isn't going to be a successful approach unless you wire in a circuit to separate the sync signals from green.