Chosen Solution
Sadly, it sounds like the displays TCon board is failing. It converts the signal from the logic board to the LCD display. This is part of the display assembly and not really replaceable. Here’s the IFIXIT guide: iMac Intel 27" EMC 2546 Display Replacement. Make sure you get the Adhesive Strips kit with the needed tool to open the system. As for the display you’ll need to take yours out and locate the manufactures number to get the correct one (Apple had two versions). To verify plug in an external monitor this should not show up on it.
gonval “a strip of the whole bottom of the screen flickers” is most likely caused by a bad backlight. It is possible to replace the backlight strips. There is also the possibility of failing backlight return filters. It will help to see what you see. If you can post some images of the flickering across the whole bottom we can most likely verify this. Use this guide Adding images to an existing question for that. Update (10/09/2017) gonval2002 if you can and if you have your iMac already apart, give us the exact model number which is written on the label on the back of your panel. It is possible that your panel has issues with the Gate drivers but most likely not a T-con issue.
