High Sierra: Second Monitor Trouble

When I upgraded to High Sierra, my second monitor began to misbehave. It had been fine for years under all the old OS versions. (It's VGA connected via my iMac Thunderbolt port w adaptor.) Sometimes it works fine, but other times it briefly shows the icons on my extended desktop and they then blink off and the screen dims to gray; I can still see the menu bar, but I can't move a window to that screen, nor can I draw a rectangle by dragging and holding on the screen. My calendar program opens on that screen and that works fine, but the desktop behind it is effectively dead. I can make this problem go away by briefly mirroring displays. It sometimes re-occurs when I wake the system from sleep.

Any ideas?