As many of you know, the iMac G5 has had a bad history of defective electrolythic caps and video chip failures. Naturally I have stumbled across a coulpe of these, one with bad caps and the other with a bad BGA soldering on the NVidia Graphics chip. The symptoms of failed BGA soldering on the VGA chip are lines on the video while booting, with intermittent dots and change of shape, always in a vertical style pattern. Now I managed to fix the this with a hardware modification instead of refluxing the BGA which is a more complex operation by increasing the pressure of the heatsink on the GPU. What I did was unscrew the 6 screws holding the heatsink. With it removed you will notice 2 supports on the top that screw the heatsink on the NVidia chip, so these are not the 4 screws with springs holding on the CPU. The brass cyilinders in which the screws go in can be filed down 1mm until they are flush with the larger diameter part (so not all of it, just file it down to the larger part of teh cylinder). You can do this easily with a large metal file. Once this has been done remember to reapply silicon heatsink grease on the CPU: don't apply arctic silver as it conducts and if you exaggerate it will overflow onto the side components and short something on the CPU. When you screw the heatsink back on, be careful while you tighten the top screws: if you filed the 2 cilinders down correctly you should be ok. At first you can tighten them 50% of the way and check if your problem has gone. if it has, apply some hot glue to avoid it getting unscrewed. Good luck!