x server crashing - sometimes

[Stephen]

Dear All

Kubuntu 8.10

I want to reduce the resolution as a way of improving the graphics and 
text size as these are to small.  I have seen these setting in two places:
1) K > System > System Settings > Display
2) K > System > KR and RTray Screen Resize & Rotate

When I chose either of these, the server exits to a login screen and I 
have to login again.  Odd, as I have used these before.
The most recent change I have made was in K > System > System Settings > 
Desktop -All Effects tab, where I unticked most of the 3D options and 
improved performance.

I would also like to reduce colour depth.  When I last looked at the 
settings for changing the resolution I did not see anything about 
reducing the colour depth.  I have looked around System Settings.

Any help appreciated

Stephen