Video Refresh Rates

Hardware:

Dell Latitude D620 w/ Nvidia NVS 110M
Dell 2405FPW (24" Widescreen)

Issue:

Typically I run at 1920x1200 @ 60Hz. At this resolution/refresh rate things
look fantastic.

However occasionally when I come out of screen saver, the system will drop
down to 1920 x 1200 @ 50Hz which, to be honest, the display really can't
handle and looks awful. (It gets squashed to the center of the screen with
about 2" margins on either side).

Is there a way to manually edit the set of supported modes for the display
so I can drop out that one mode from the list of mode the monitor
'supports'?

Thanks,

-- Paul