My Mac shuts down when I downloading the software update *flickers too

My Mac shuts down when I downloading the software update *flickers too

A few weeks ago my Mac started flickering uncontrollably. I thought it needed a software update since I haven't updated the software for a while. While the update is downloading, at the same time my screen flickers. After the update is finished the Mac shuts down. I switched on the Mac to find the update restarted the download and my screen still flickers. Can I fix this