Do you simply move the mouse cursor away from the game window or you also activate other windows?
There is a parameter in psclient.cfg that you can try to modify:
Video.SystemMouseCursor = yesChange it to
no and see what happens. At least it didn't harm my system

Otherwise I am afraid that you have to wait until ATI releases a new driver that hopefully fixes the problem. I had some random freezes with my NVidia card, but they got fixed with a driver upgrade.