Fog Effect etc. Problem

Hello,

after getting my mage to level 80 I went back to my crew and ran into a problem after about 10 min.

At Garroshs Landing in Borean Tundra the screen turns to a fog effect... on my 2nd (slower) pc with two clients this is no problem.
But on my main pc with 3 clients two windows just turn black and lag really, really bad.

Ok... I can skip that area but the same effect is also used later and I don't want to skip some instances (CoS) and areas.

Someone has a idea how to fix this? Or do I need a new graphics card? I'm currently using a 7800

thank you all