I wouldn't be so quick to attribute this to a hardware problem if the system has a Radeon HD2000-series GPU. I've triggered this corruption (as well as other GPU driver failures including kernel panics) by running certain applications. I also experienced graphics issues in OS X 10.4 as well that went away after installing OS X 10.5.
Are you running
10.5.6? It includes these graphics fixes:
- Includes general improvements to gaming performance.
- Includes graphics improvements for iChat, Cover Flow, Aperture, and iTunes.
- Includes fixes for possible graphics distortion issues with certain ATI graphics cards.
Previous OS X 10.5 patches have included significant graphics-related fixes and improvements, including some specifically for ATI GPUs. In other words, the drivers weren't very good to begin with.