It started of with stripes (horizontal lines mostly) on my 30 inch Mac monitor; next came freezes out of “nowhere” it seemed, often occuring when a heavy graphic duty needed to be performed (like the unnecessary but elegant animation of a window on your screen being put into the Dock); then more freezes especially while using Adobe Photoshop 3; etc..
The culprit turned out to be the graphic card: ATI Radeon X1900 XT . The reason overheating of the card, or better said bad cooling. Not enough air coming in, so dust.
Opened up the G5, made from a piece of cardboard a sucking device that fitted my home vacuum cleaner and took out an awful lot of dust. This helped a bit; but soon the problems reoccurred and this time more frequent. Now this is the lesson: I started to look for other causes than the graphic card… spending a day or so on this exercise. That was wrong. The card must have been damaged because of overheating in an earlier stage, still functional up to a certain level and I mislead myself looking for another source of trouble.
The “proof” is when the earlier mentioned freeze midway of the animated trajectory of an open window on the desktop to the Dock halts midway. That is a hardware failure of the graphic card and nothing else.
I ended asking my dealer to get me the same graphic card to test is and YES all problems were gone.
One of the sources that put me on track was at Apple Discussions…
the limping messenger 
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