external monitor with iMac

For several years I used an old 19” Acer monitor as a second display, attached via its VGA port to my iMac (2011 model). After I bought a iMac 2019 model, I again used the same monitor attached via its VGA port to my iMac 2019 with a lightning to VGA adapter cable. 

This worked well for about 2 years, but abruptly the monitor screen began to flash off then back on, not in a regular pattern but on average several times a minute. When it did so, the iMac screen was affected in a similar though not identical way, keystrokes begin to lag, and sometimes the whirling beachball starts whirling very fast.

I could not find out what had gone wrong, so I disconnected the monitor and bought a new Acer LCD monitor—model K202HQL . Today I attached it via its VGA port to my iMac 2019 with a lightning to VGA adapter cable. To my dismay the exact same problem happened right away. This tells me I am doing something wrong, but what? The iMac continues to run OS 10.15 (currently 10.15.7). When the Acer is disconnected, the iMac performs well.

Any ideas on what I can do to fix this?