Yes wrote:

Within the last few weeks, the screen will go blank literally for a
second or two when I'm using my pc. I'll be at a web site, and all of
a sudden the screen blanks (turns dark), no error messages. A second
or two later, the image reappears as if nothing had happened. AFAICT,
there is no interruption in the stream from the site nor any shutdown
of the browser (Firefox latest version) itself. It may happen once
every several hours. I am not familiar with this type of problem, so
am asking for suggestions about what the problem might be. Please keep
suggestions simple - my skills and interests are limited to replacing
stuff, not equipment testing.

The motherboard is an ASUS B150M-A bought in 2016. The CPU is an Intel
i3-6100 CPU with stock cooler and 8 GB RAM (Corsair DDR4-2133 and
compatible for the m/b). I do not have a graphics card. The built-in
video component of the CPU has been sufficient for what I do with the
pc - surf the net. I have not noticed any problems with boot up or
with shut down. The O/S is Windows 10 Home edition. That's it for my
pc.

If the video cable is not integral to the monitor (i.e., it connects at
both ends), try a different video cable. Got a friend with a spare
monitor they can loan you? Could be the backlamps or inverter in your
monitor is going bad.

Have you checked the video cables? I've seen where they were just
pushed on (although they had tie-down screws but were loose or unused),
and eventually any torque on the cables, like pulling on a mouse cord
that touched the video cable or banging fists on the table caused an
intermittent connection to the monitor.