X not working on AMD64 Radeon after upgrading 9-Stable to 10-RC3

[Graham Menhennitt]

I have a box that's been running X and Kde4 on 9-Stable successfully for 
ages. I tried a source upgrade to 10-Stable but couldn't get X to run 
properly. I then blew the old installation away and did a fresh install 
of 10-RC3. It has the same problem.

I start HALD and DBUS and then start X (in various ways - see below). 
The box then dies - the screen goes black and the monitor says "no 
signal". I can't kill X with Ctrl-Alt-Bksp, and I can't switch back to 
the console using Alt-F1. I can log in remotely via ssh and see that 
Xorg is using 100% CPU time. I can't kill the Xorg process (even with 
kill -9). All I can do is reboot. I've tried with both a monitor 
connected to the VGA port and a TV connected to the HDMI port - both the 
same. If I add "kdm4_enable=YES" to /etc/rc.conf, then that same hang 
happens soon after boot.

The box is AMD64 with ATI Radeon RS690 X1250 IGP. Dmesg output is at 
http://menhennitt.com.au/starker/dmesg. The config file generated by 
"Xorg -configure" is at http://menhennitt.com.au/starker/xorg.conf.new 
and the log from running that configure is at 
http://menhennitt.com.au/starker/Xorg.0.log_configure. Running "X 
-config xorg.conf.new"  as root produces the log file 
http://menhennitt.com.au/starker/Xorg.0.log_1.

If I run startx as a non-root user, I successfully get a Vesa VGA 
session. The log file from that is at 
http://menhennitt.com.au/starker/Xorg.0.log.

I've done a bit of googling and found a few people with similar 
problems, but no solution. Can anybody please give me some suggestions.

Thanks in advance for any help,
     Graham