I've had this problem for a few months now:

-I can use iMovie without problems until I try to "export to iDVD".
-When I try to watch a movie DVD with Apple's DVD Player, the lower half of the image is corrupted for a few seconds; then it crashes.
-I have no problems watching movie DVDs when I use another program (VLC).

The crashes are severe: even the pointer freezes.

I tried to reinstall the system from scratch. I used the disk that came with the Mac (OS X 10.2). It crashed on all my attempts, during installation. However, I installed OS 10.3 (bought at the store) without problems. I don't know if these issues have anything to do with the video issues.

Here's my equipment:
Dual G4 (FW800) 1.25GHz, 1.5GB RAM (factory installed), OS X 10.3.9. Factory installed graphics card: ATI RADEON 9000 pro, 64MB.
Recently (long after problems started), I have installed two things: an internal Airport card, and a USB 2.0 PCI card.

Does anyone know why the DVD (MPEG2) codec crashes, but not so when I use another app? Could it be a problem with the video card? (Photoshop, iPhoto, etc. work fine). How is the MPEG2 coding/decoding done: Is it hardware, or software?

I would appreciate any help...
Felipe Sanchez-Cuenca