gdm gives me a flashing login screen

[dave <dave lake demon co uk>]

I upgraded (!) RH6.1 to 6.2, and I now get a flashing login screen for
gnome. xdm works fine. My gdm conf file hasn't changed, so I'm very
puzzled
The screen actually flashes as if it starts up, crashes then starts
again. It comes up about every two seconds, with an associated
'resolution switch' noise from the monitor. Here is a snapshot of my
processes while this is happening.


100     0  2464     1   1   0  2592 1016 do_pol S    ?          0:00
/usr/bin/gdm -nodaemon
100     0  2542  2464   9   0  8980 2336 do_sel S    ?          0:00
/etc/X11/X -auth /var/gdm/:0.Xauth :0
140     0  2543  2464   5   0  2596 1124 wait4  S    ?          0:00
/usr/bin/gdm -nodaemon
000     0  2547  2543  10   0  1648  752 wait4  S    ?          0:00
sh /etc/X11/gdm/Init/Default
100     0  2550  2547  17   0  2928  984 -      R    ?          0:00
/usr/bin/xsri -geometry +5+5 -avoid 300x250 -keep-a
100     0  2551  2397  17   0  2700 1000 -      R    pts/0      0:00 

Can anyone help?

many thanks
Dave