CentOS - Nov 2017 - /usr/bin/x0vncserver large blocks on screen at times

I use this vnc program to remote into my station. (C 7.4)

When I do, about every 5 to 10 seconds a big solid block appears on my
screen. About six inches long and 3 inches wide. it quickly then goes away.

I use the realvnc.com viewer when I do this.

Is there some setting that would help with this ?

Jerry