iMac G5 Startup Problems

I was not able to see the video link.

However, if you have not already, you should try resetting PRAM

http://support.apple.com/kb/HT1379

and resetting SMU (power management)

http://support.apple.com/kb/HT1767

When you connect the power cord, if you were using a power strip (surge suppressor) or some type, try connecting it directly to the wall outlet, by itself. Initially power it up with nothing else connected, then add only a standard mouse and keyboard. Use it that way for a while to see if it is any better.

Graphic chip or more likely its solder gone bad. In some cases it can be bad capacitors too, either on motherboard or power supply. Cannot be fixed with software tricks, in some cases the machine might work just fine if you use it in safe mode or remove graphic chip related kext -files (disable acceleration) but that's not really a fix. In your case it seems worse as you have those artifacts right from the beginning.

If it is graphic chip related then there are two fix methods, either reflow the old solder back or reball it by using solder which contains lead. Of course these require a professional who knows how to do it. In some cases it is really the chip which has gone bad but in most cases the connection between motherboard and the chip is not reliable anymore.

Well power it up and press shift key to start in safe mode, if you still see those artifacts then you can be sure it is hardware problem. However, like I said, in some cases you might be able to run the computer without acceleration (=safe mode) just fine but as soon as you boot in normal mode (=acceleration on) there will be problems.

You could also try booting from OS X installation disc, if you have those artifacts with it also, then you can rule out all software problems once and for all.