Auto Login Failure

On a fresh install of Ubuntu 9.10

I've set the auto-login at 10 seconds seeing as the machine will only 
ever have one user.

Sometimes the auto-login works - some times it doesn't.
When it fails,  the login does its 10sec countdown, screen goes black 
and then login panel reappears as a manual login.
When you then type in your password it keeps looping back to the login 
screen.

Restarting the pc fixes it - for a while.

Where or how could I find any error messages related to this?

cheers
chris