Several freezes...

[Radada]

Hi there

Since I started playing Penumbra BP, I had several freezes. After that, nothing respond. I cannot get the "Force to quit" window, I cant get out of the game, I can't minimise it. The only thing I can do is a hard reboot of my imac, which I hate to do.
I join the hpl.log file.
Thanks for your help.
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EDIT : I have a 24' iMac (first generation, NVIDIA GeForce 7300 GT) and leopard 10.5.2.