Chosen Solution
I have an Early 2009 iMac 24 Aluminum (2.93GHz) and had it in a box for a few months, then started using it again after moving into a new house. When I powered it on, after booting up, the screen starting going black then back on, then black, then back on. Quick flashed of the screen turning off. I randomly decided to lower the brightness down to 40% and it has not been flickering anymore. Any thoughts what the flickering is from? Surge protector? After market RAM that was installed recently? Graphics card?
Sounds like a graphics or controller issue. Connect to an external monitor. If the behavior is there it’s part of the VGU/VGC, if it’s gone it’s a component in your internal display chain. If this Answer is helpful please remember to return and mark it Accepted.
My “solution” is different: If the iMac is cold, the screen is black. No problems on the attached monitor, so I set a movie going in VLC and soon the iMac is warm enough to reduce the flickering.
re the above answer: “If the behavior is there it’s part of the VGU/VGC, if it’s gone it’s a component in your internal display chain.” Does that mean that if there is no problem with the external, its #”a component in your internal display chain”? BUT what does that mean?
Same happened for me. Lowering the brightness worked
