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The GPU is damaged. Take the iMac to an Apple Store or reseller to get it repaired, but the logic board has to be replaced, so this may be an expensive repair.
If you have a very old Mac, Apple may not have parts to repair it. In this case, the best solution is to get a new PC or Mac so you can use the most recent operating system and you will have a PC or Mac that will be useful for years instead of using Tiger, launched in 2005