Mouse look movement stutters or jerks irregularly when holding keys down. This effect becomes much more pronounced at low frame rates.

The first example is with the frame rate locked at 10 and shows how the mouse look input is uneven and jerky when holding down shift to crouch:

https://www.youtube.com/watch?v=3f7FDMI2yQE

Here is an example with just moving the mouse in a small circle with the frame rate locked at 30 fps:

http://youtu.be/985aI-JfXUE

While a key is held down the sensitivity drops in a repeated cycle, causing the view point to spiral inwards when moving the mouse in a circle.

The issue turns out to have been in LWJGL, and has been fixed in 2.9.3. If Minecraft updates to use LWJGL 2.9.3 instead of 2.9.1 the problem will be resolved.