I had some stuttering problems in 2.4 with multiplayer traffic but this has been greatly reduced in 2.6.
Then I had serious stuttering in complex scenery (lots of vertices) but this went considerably better when I replaced my GT520 with a GTX560 GPU.
I still experience stuttering with 3D clouds but as Thorsten explained in another thread, this is hardly avoidable. Further stuttering I experience on turns, when lots of new and rather complex objects come into sight. Think e.g. LOWI and take a helicopter and turn while hovering. I think, this is caused by the data not being pushed into the GPU fast enough. I'm not sure if and how this would be solveable - maybe by using more DDS textures but this is just a wild guess without deep knowledge of 3D graphics stuff.
There is a thread about stuttering here. Maybe it would make sense to join the threads together?