xubuntu 8.10 crashes on X logout

[Chris G]

I have a new Intel chipset/graphics system (Asus P5Q-VM motherboard)
system on which I have installed xubuntu 8.10.

It all installed OK without any problems and ran OK.  However since I
set the graphics resolution to 1600x1200 (the display's default) it
crashes *totally* on exiting from X.  It's a total, hard, crash -
CTRL/ALT/Fn won't switch to a text console, I can't ssh into the
system (which I can before it has crashed) and I can't ping the
system.

There is one other symptom, when I try and switch to a text mode
console (using CTRL/ALT/Fn) even when the system is running OK I get a
message from my monitor saying "this mode isn't supported" which seems
a little odd on a monitor capable of 1600x1200 at more than 60Hz.

Is there any way I can try and diagnose this problem, I have looked in
/var/log but I can't see anything useful in any of the log files there.

Any other ideas?

-- 
Chris Green