Exactly! That's it!
I've only tomorrow enabled better graphics (including, but not limited to, shaders) and I haven't been out of Hydlaa since then, so I've no idea whether enabling shadows fixed my problem.
it's not related to shadows - at least not according to the tests I did on that card as I had it.
it's been strictly limited to the HD 3200 for me (i.e. did not occur with my other cards (HD 4200, HD 4830, HD 5770, GeForce 6800 GT)), so I suppose it's some issue in the graphics driver or with the card itself.
I don't know whether or not this is fixed in the 10.x drivers or can be worked around by using the open source driver instead of fglrx, so some testing may help.
However it is an issue with the shader implementation for those cards (i.e. nothing we can fix on our side, but may be fixed by fallback shaders specific to those cards on CS' side - unfortunately there aren't enough ressources to write those).
Anyway, the issue isn't that big, so I hope you still enjoy the game
