[Hampshire] Gnome / ubuntu issue with login / logout

Heya,

As per slug, I've just completed a fresh installation of Ubuntu - hardy
heron.

I'm still stuck with one issue on the desktop though which I can't
figure out. It was the same in gutsy, even though it's been blitzed.
I'm assuming it's a hardware incompatibility in that case but...

I have auto-login enabled, and it will sometimes (about half the time)
only boot to a brown screen (pre ubuntu 'sound' which I haven't nuked
yet ;)). I cannot use Ctl+alt+F% to switch to a terminal, however the
mouse moves on the screen. Luckily it responds to a hit to the power
button (only since hardy!) and I can start over and keep trying in a
russian roulette fashion until it boots in successfully.
Also when shutting down a similar problem occurs, it hangs when bringing
up the shutdown / reboot dialogue and I have to use Ctl+alt+F% to power
down manually.

This is all under gnome, any advice appreciated - hours of googling and
trawling logs has got me nowhere!!

Thanks,
Charlie