What video card are you using? Is it the nVidia card that sometimes came with the G4 DA's, or is it an ATI card? I had similar issues when the nVidia card on my DA began taking a dump. Swapped in a Rage 128 Pro that I had from a parts G4 DA. No problems. Eventually replaced that card with a reflashed Radeon 9700. I didn't have any issues swapping my original 533MHz processor with a 1.2GHz G4. Computer continued to run normally. Just a lot quicker.

-J