>> The flashing screen problem indicates your monitor cannot support the 
>> current screen resolution.
>
> Not these monitors, they print a nice text message. I have a DELL 1801FP
> (on that computer) and a DELL 1901FP on my KVM. Both are fairly good 
> LCD monitors.
> 
> Especially when it's often displaying something, when flashing.
> 
> The flashing is most likely a buggy driver.

Just to confirm this, I would recommend navigating the "menu" buttons on the monitor(s) until you find the "information" or "mode" data, which will report (on the LCD screen) "Hpixels x Vpixels x V-Refresh" to which each monitor is trying to sync.

Back in the glory days of CRT monitors, best (flicker-free) performance was found at high vertical refresh rates (e.g., 85Hz).  But with LCDs, at least in my experience (mainly with Viewsonic VP171b LCD displays), 60Hz vertical refresh rate seems fine.

If the LCD monitor menu information indicates that X is trying to "over-achieve" by working at a higher vertical refresh rate than the minimum necessary (say 60Hz), here's what I'd do ...

Multiplying Vpixels times V-refresh gives a lower limit on the horizontal sync frequency needed to support that video mode (it's the "lower limit" because it includes no overhead for anything besides actually refreshing lines on the screen) ... you'd probably want to round upwards (by at least 5%, maybe 10% ?) the result of the multiplication (or check what the LCD display itself reports in that mode when working properly on a Windows PC :-).

Anyway, where I'm headed with this is:  based on the Vpixels x V-Refresh mode that you want from SL5.1 (for 1280x1024 say, 1024 x 60Hz = 61Khz ... call it 64KHz), you might try reducing the monitor top vertical frequency and top horizontal frequency capability in the /etc/X11/Xorg.conf (or XF86config-4 ) file, to just slightly more than necessary for 1280 x 1024 x 60Hz, to prevent X from trying to over-achieve in frequency terms ... 60Hz vertical should be less demanding on video card RAM etc., compared to 72Hz or 75Hz or 80Hz or 85Hz.

For example:

Section "Monitor"
     Identifier    "Monitor0"
     VendorName    "Viewsonic"
     ModelName     "VP171b"
     HorizSync     31.5-64     # units are KHz
     VertRefresh   50.0-62.0   # units are Hz
     Option        "dpms"
EndSection

Also (whether or not you find any value in the above suggestion), have you looked in the /var/log file for X startup for any clues about what might be going on ?

Best of luck,

     -- Jim