Chosen Solution
This iMac have a damaged vsync cable. it can only boot up with safe mode, without safe mode it will always stuck at the apple logo after the chime or load up a cursor then became the beach ball. If it idle long enough in this state, the display will go to sleep. When in safe mode, I can’t run cinebench to test the gpu in safe mode. If I try to browse the web, Safari will flicker non stop, the video is just black colour. If I connect a external monitor and drag safari to the external display, I can view a flickering image of the video. If I use safari in recovery mode, it flicker a lot lesser but stuck with beach ball and flickering stop. Is this more like gpu issues or I just need to replace the vsync cable?
You’ll need to start off replacing the Vsync cable. Your symptoms are not at a point which tells us enough. I would also recommend you try either booting up on an external drive or try running in Target Mode connected to a second Mac to make sure your drive is OK.
