Here are couple of mods which may help in low fps issues or just make the fps behave steadier.
The first one is Framerate Adjuster. The addon adjusts the distance terrain is drawned based on your setting. If for example you in average have an fps of 30, you can set this addon to drop the drawing distance in cases your fps drops below your set limit. When the limit is easily achieved, the addon will again raise the distance so you can see more of the world in front of you.
The second is GCTweak. Again based on your fps-limit which you can set your self, GCTweak finds opportunities to run the Garbage Collection (a process that attempts to free memory, also abbreviated as GC) more often than Wow in default does. Framerate should be set to your normal fps situation so the extra GCs don't run when resources are low anyway.
You might think extra GC's aren't a good thing, normally GC is an expensive operation to do. The thing is: It's more beneficial to run many small GC runs in stead of large GCs seldom. Since the extra GCs are performed only in good conditions (fps>=set limit) there shouldn't be a noticeable effect on your gaming experience. Extra GCs also help the regurlar GCs as when the GCs are performed more often, the memory usage wll stay smaller and this speeds up each GC.
Note: There is something similar to GCTweak in the plans for patch 2.3 - the details are not clear at this time. 2.3 may make GCTweak obsolete or not, Framerate Adjuster should be viable after that as well.