Try running the game as administrator, and in the compatibility options disable desktop composition (in Vista/7).
Did you go through the graphics options and try making changes (ie disable HDR lighting) to see if something would help? Someone mentioned disabling the option to "render shaft lights" to get rid of the bloom effect; it sounds like that could be what your system is having a problem with.
Did you check ATI's site for updated drivers?
You may have seen these topics already (and they don't mention this particular problem), but
First Aid Kit for Divinity IIFirst Aid : when Div2 does not start or crashes