It looks like i had a kind of hw issue. Some time aog i found that a cooler on my video card did not work properly.
> Damir R. Islamov wrote:
> > Package: xserver-xorg-video-ati
> > Version: 1:6.9.0+git20081129.783cdb73-1
> > Followup-For: Bug #507254
> > I´m able to reproduce the bug with different snapshot of the driver:
> > git20080827.38aa450f
> > git20081129.783cdb73
> > git20081214.f9826a56
> > With all of them my system hangs. However i can access via
> > ssh and reboot it manually.
> > Upgrading to experimental/downgrading to unstable gives the same results.
> > Playng with EXA<->XAA changes nothing.
> > I should add that i can observe all symptoms from
> > #490071, #479870, #462590, #461580(except ssh).
> Your X seems to be using AGPMode 8. Maybe this mode isn't actually good
> for your box. Can you try AGPMode 1, 2, 4 instead? Or even add Option
> "BusType" "PCI"?