In-game stuttering occurs when the game skips frames because it did not receive the data fast enough. Stuttering does not affect frame rate, thus you cannot "see" if you're stuttering simply by looking at the FPS counter.
The most common causes of stutter are:
- High memory usage (swapping) - Spikes in CPU usage - Limited GPU bandwidth
The first two causes of stutter can usually be remedied by closing some background applications.
The third one is mostly a hardware issue as it is common with low-end GPUs.
Here is an interesting link to help you understand the concept of GPU bandwidth.
Tuning down your textures and your shadow settings can help mitigate this issue.