Just like to add to this topic that I was having a similar issue where I was running around with 70-80FPS then for no reason the game would drop to 20-25fps. If I ALT+Tabbed out and back in again immediately I would get 80FPS again for about 10-15 minutes till it dropped. This was looking at the same spot in the game without moving.
The issue has been fixed for me by downloading the AMD Catalyst Application Profile 12.8 CAP2 (Note: Installing the 12.8 Catalyst drivers did NOT fix the issue, i had to manually install the CAP2 before the problem went away)
Sysetm: Intel i7 2600k, 16GB RAM, 256Gb Revo, ATI HD6990
As someone else mentioned the CAP2 specially adds a profile for Guild Wars 2 to fix FPS issues.