Some Power Mac G5 computers equipped with a Radeon X850 video card may exhibit distorted video when waking from sleep, using certain screen saver modules, or when a graphics-intensive application is in use.
Here's an example of what such distortion may look like:
To resolve this issue, update to Mac OS X 10.4.3 or later. You can use Software Update to download and install the update, or download it from Apple Downloads.