freezing problems plus unfamiliar screen symbol
Yesterday my iMac started freezing. It continues to do so today; now it seems, most particularly after it has gone into screen saver mode. I have rebooted in recovery mode and found no problems that needed repairing; then I did a hardware test and again found no problems. But I still have freezing problems (plus significant beach balling problems) and have also had it freeze when in screen saver mode into a black screen with a white circle with a white slash through it in the center of the screen. See attached.
Can anyone tell me what the screen symbol is trying to tell me. I can force shut down and restart when that happens.
I updated the Software this morning, but still have problems.
Thanks.
iMac, macOS High Sierra (10.13.6), 3.4GHz i7 (Fusion) 32GB RAM