The symptoms you’ve mentioned are a-typical of the common graphics fault on the 27” 2011 iMac ranges unfortunately. The cause of the common fault on this range is related to the GPU, which is removable on this range.
Basically, they used lead free solder for the GPU that made it more environmentally friendly, but with the casing design on these machines holding in so much heat, and the lower melting point of lead free solder, the GPU joints would fracture. The main reason it sounds like its the common problem is because it starts out intermittent, and is most affected when the machine is hot. So the multiple reboots would function the same as leaving it to cool for a while before re-trying.
The only other possible cause is LCD panel or logic, but GPU is most likely.
You noted having tried a replacement GPU already, but was it a re-balled card, or one with the original lead free solder? as you cannot buy these cards ‘new’, it is likely second hand, and very likely suffering from the same fault.
we only ever fit re-balled cards (not re-flowed cards) because of how common this faulty is.
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Can you post a few pics so we can see what you see on the screen.
Also have you tried an external monitor to see if the effect is on it as well.
von Dan
All you need is a Mini DisplayPort cable to either a VGA or HDMI connection needed on your monitor. As example Mini DisplayPort to VGA Adapter
von Dan