Thanks for the replies.

Well, it's not the monitor, as I've been using the same monitor for the
past couple of years.
Only one other person normally accesses the computer... with no
symptoms. Then again, I'm much more sensitive to the ozone from laser
printers, than many other people are.

I removed some "excess" internal peripherals, to see if increased PC
airflow will help.
EG Maybe a ribbon cable is touching against something hot and will no
longer do so, with more room in the case... This is the first PC I've
had that did this! I had a Thinkpad for work and it was totally fine...