[CentOS] system-config-display problem

I bought a Viewsonic VG730m monitor, analogue and digital.  With a digital 
cable connected the screen keeps blanking for a second or so.  
system-config-display shows it configured as a CRT monitor, and refuses to 
accept that it is an LCD monitor.

I've tried manually editing xorg.conf, putting in the manufacturer's values 
for horizontal and vertical refresh, but the problem remains.  The Viewsonic 
website has lots of information for windows and some for mac, but nothing for 
linux.

I need either

to find how to make system-config-display keep the setting I'm giving it

or

to know what entry to put into xorg.conf to make it understand that it is an 
LCD display.

Sorry if this sounds naive, but I've not had a digital monitor before.  I 
could go back to using the analogue connection, which gave no problems, but 
you'll understand that I'm reluctant to do that.

Anne