How to increase FPS in WoW and other games like 3D?

Ok umm i have like 60 fps and osme times 40 and 30 in 3d games like WoW guns game and such. When i continue playing my fps goes to 5 randomly for like 3 min and goes back to 60 and 40 for like 30 min and goes bakc to 5. How can i fix this? i tried going to the nvidia panel and changing settings and my power is on high performance. how do i make it stop going to 5 fps randomly? thank you also the settings on wow is low