We had a user with a constant 60Hz flicker on his crt. Wasn't a nearby fan. Swapping monitors didn't fix it. The same monitor worked fine elsewhere. Changing settings just changed the patterns.
I was totally stumped until I realized that the ginormous breaker panel that fed half a factory office floor was directly on the other side of the wall. The user simply rearranged his desk with the monitor on the opposite side and all was well.