I have an ATI card at the moment (HD4870) which is rather old but I haven`t had any major problems. A programmer who I know in Poland who is working on updating Drakan to work on modern systems and add extra DX9 features had a load of problems with ATI cards. He said it is due to the card's internal rendering processes which are significantly different to Nvidia and managed to find a solution. Could be something similar with Drakensang. I played the first game on the card and it worked fine but not tried Drakensang 2.
Does sound like a driver issue or setting. ATI / AMD are known for buggy drivers on newly released video cards though and it tends takes a bit for them to make stable drivers. Not had this myself though.
What card have you got?
