I have thoroughly cleaned every aspect of this iMac. There is no dust left. I put new thermal paste on both the CPU and the GPU. Using SMC fan control I have increased the CPU fan speed (which also cools the GPU), and the artifacts have been significantly reduced. Not gone, but much better than before. Initially when I boot the computer or return from sleep mode there are several minutes before anything shows up. I think Apple can fix this issue, and it would not cost them a whole lot. If they made an optional flash utility for these computers (my research is showing this as a very common problem), which reduces the clock speed of the core, I am sure the issue would go away. Sure our graphics would be a bit slower, but that is a lot better than an $800 logic board replacement. I once had an iMac G4 800 which was not stable until I used it in the garage during the winter. I found the PLL control resistors on the logic board and cut the iMac down to 700MHz. It was perfectly stable after than even in high ambient heat.