External Monitor "desktop" issue

I have an iMac - Retina 5K, 27-inch, Late 2015 running High Sierra.

I use an external monitor for an extended desktop. (It's an HP model that I turn sideways so it is in the "tall" position)

Sometime about 6 to 12 months ago the "desktop" of the external monitor would always become unresponsive when waking up from an extended sleep period (anything more than a few minutes). Desktop icons disappear and the desktop is unresponsive to everything. And app window on that monitor still works fine.

Unplugging and reconnecting the connection cable will bring the desktop back to life.

I've tried thunderbolt to HDMI, and thunderbolt to SVGA (?) cables. Same results.

Two MacOS updates ago this problem was fixed! My external monitor worked flawlessly for about 2 to 4 weeks... then the latest update (10.13.3) broke it again.

I've tried repairing permissions (using TechTool pro) and many other system cache cleaners and such...

Any ideas? Anyone else have this problem?

Randy