The speakers were removed completely and all electrical equipment except for mouse and keyboard were relocated about 4 feet away from the monitor.

A few years meaning I've had the speakers and enclosure near the monitor (within 2 feet) for about 6 years.

The monitor was also connected to another computer.

The end result is the same.

I've also tried dropping the refresh rate to 75Hz and using different 4:3 resolutions. IE 1600x1200, 1024x768 etc. The concave is gone, but I notice on the left and right sides of the screen, it squishes in.

I suppose this means I have to replace this monitor?