How can I get DVI digital signal to work?

I have a Samsung SyncMaster 740B LCD monitor and an ASUS PCI Express
graphics adapter with NVIDIA 6200 chip.

The graphics adapter has VGA and DVI-I output, and the monitor has VGA
and DVI-D input.

If I use VGA, everything works fine. But it's hard to get DVI digital
signal to work, it works sometimes, but not always, and it seems random
to me when it works.

Sometimes I get DVI output from the BIOS startup messages, but usually
not.

I run Linux and I have configured X to use DVI if possible,
Option "UseDisplayDevice" "DFP"
in xorg.conf.

Sometimes X uses DVI output (more often than BIOS does), but sometimes
it doesn't and I get this error message:

(WW) NVIDIA(0): Option "UseDisplayDevice" requested "DFP", but no
unused DFPs
(WW) NVIDIA(0): are available.
(II) NVIDIA(0): Option "UseDisplayDevice" "DFP" converted to "".
(WW) NVIDIA(0): Unable to find any of the requested display device ""
in the
(WW) NVIDIA(0): list of available display devices "CRT-0, TV-0".
(II) NVIDIA(0): Assigned Display Device: CRT-0