How do I prevent freezes or FPS drops when in combat?
Seeing as you said the fan no longer spins properly
The more the video card has to render, the hotter it will become. The hotter it becomes, the more it relies on its heatsink and fan to disperse heat. If the fan is not spinning properly, you will have all sorts of issues trying to render graphic-intensive scenes - as you have described in your comments above.
Lubericating the fan may not solve your issue as you may have a problem with the bearings in the fan.
I would recommend replacing the fan on the graphics card or replacing the entire graphics card altogether.