Apple Performa 6400/180 - Manual (Page 44)

Basics 6500 Supported Monitor Resolutions - 42
6500 Supported Monitor Resolutions
The system software on Power Macintosh 6500 computers
allows selecting monitor resolutions above 1152 x 870. But
these higher resolutions cause poor video output, such as
misaligned or skewed video, low brightness, and so on.
Monitors connected to Power Macintosh 6500 computers
should not be set to resolutions above 1152 x 870. Check the
Monitors and Sound Control Panel to make sure the
resolution is set to 1152 x 870 or below.