Ridiculous FPS drops

My game runs fine generally speaking (60 fps). When certain spell effects occur, however, it drops to 1-10 fps, sometimes for several seconds. Playing in groups is impossible for me, because I can only control my character 50 % of the time. My videocard is an ATI Radeon 5700, I ve also heard of other people having the same problems with ATI cards. Any suggestions on how to fix this? If not, I just want to bring it to the dev teams attention, seeing as this is probably a huge annoyance to a significant amount of players.

Edit: It's also happening solo, especially in act 3 where mobs cast a lot of spells. Game is nearly unplayable, such a shame because i enjoy it so much otherwise