Freezed desktop after log in
I've got some problems with Ubuntu with FX5200: when I turn on my computer, sometimes I can log in Ubuntu, and sometimes not because after logging in, just before showing the desktop, it gets freezed and I just can move the mouse or switch to console or ctrl + alt + backspace. I have to reboot my computer until it gets booted normally (sometimes 3 or 4 times). I tried dpkg-reconfigure xserver-xorg but it's the same. I thought it was a graphic driver problem, but I tried and ATI Radeon 9550 and it's still happening EXACTLY THE SAME, so I'm out of ideas.
I would really appreciate your help.
Thank you very much.