what causes distrotion on graphics
The Screen on the IMac G5 has a checker like lines as if the clarity or the line smoothing has issues.
iMac
Either the Graphics Card is going bad, or the capacitors on the logic board are going bad.
If you have a non-iSight G5, you can easily remove the back of the computer, and see if the capacitors are bulging, or leaking, or if there are dark spots on the top of them.
I have/had a simliar problem with my G5 ALS, and my graphics card is the problem.
If it's the capacitors, they can be replaced by a good technician, costs between $250-$350 dollars.
Google 'iMac G5 capacitor replacement'.
Good luck.