moire on monitor - why?

A couple months ago I noticed that my 5 y/o HP CRT monitor has what
appears to be moire on the left side. In addition, the left side of
the screen will sometimes appear to be moving left and right a couple
times very briefly.

I've had no other problems with this monitor although it does now have
burn-in despite using a screensaver.

Is there any reason why the moire effects would appear and why the
screen appears to move?

Could be interference on the DC power rails inside the monitor getting
HF ripple from the switch mode power supply... or line ripple from the
fly back generator stage, the fly back stage not only generates line
scan energy it also generates other voltages for various circuits,
usually due to electrolytic capacitors failing.

Another cause could be external interference try moving the monitor,
may be worthwhile checking the signal cable as well.

Trying the monitor on another unit or trying a different monitor will
confirm the blame.

Davy