I have a late 2015 27” iMac that has been a workhorse for the last couple of years. I was working on it this morning, then rebooted and it took about three minutes (very unusual). When it finally loaded the login screen, the display loaded in sections, like horizontal lines (almost like Venetian blinds opening). It does this a few times, then flickers whenever I try to do anything on it. Documents open but load in horizontal lines. Videos don’t play, and when I press the volume key, there is a symbol indicating that there is no audio. The screen often flickers too, depending on what I’m doing, and the fans have fired up on and off even when on the desktop. Hard drives have plenty of space and this is a very well looked after machine. Could it be GPU related?
Any help greatly appreciated, before I box it up and drag it to the Apple Store.
Many thanks in advance,