Demo crashes at Ubuntu 9.04 x86_64 (ATI card)

[Przemulala]

Hello!
I downloaded and installed demo of Penumbra: Black Plague at my fresh installed Ubuntu 9.04 x86_64. I've got ATI HD3200 graphics card with the newest x86_64 drivers (Catalyst 9.5). My computer has also AMD Athlon 64 X2 4850e processor (2.5GHz) and 4GB of RAM.
My problem is that after about 10-minutes play system crashes. Before this I've got no problems with the game, it works very well and nothing seems wrong. But then it catches freeze frame and I can see mouse coursor for a short while - anything I can do is just restart it (Alt=D and Ctrl+Alt+Backspace don't work)...
What's wrong? What I can do with this? I want to buy a full version but if it is going to be like this I won't.
Thanks for all help!

PS: Sorry for my English, it isn't my native language.