OK well check out your XF86Config-4 file.

/etc/X11/XF86Config-4

Log in as root and -check it out-log in as Knoppix-check it out. You could copy and paste to an email draft or save it to a text file for example just so you can have the two side by side and not have to constantly switch back and forth. The things I would look for are:
Under the "monitor" section
-HorizSync
-VertRefresh
Under the "device" section
-which driver is it using vga, svga, (mine's a savage for example)
Under the "screens" section (and here's where I would guess the problem to be)
-default depth
-are the modes the same under each SubSection "Disply"
Obviously if it is set to a default depth of 16 then you aren't utilizing your monitor very well- change the default to 24 or 32.

Post your XF86Config if you need help.