Thanks for your reply.
I've updated and also reinstalled the most recent drivers. I've tryed some of the older versions as well but non of them seem to improve the situation.
I guess I'm just going to try some more driver versions.
I also found this but non of the suggestions there made it any better for me either and since it's an problem only occuring if I use Tekkit, I thought it might be some problem within the code of it.
Sagulatus