Minecraft 14w06b, Minecraft 14w07a, Minecraft 14w08a, Minecraft 1.7.5, Minecraft 14w11b, Minecraft 1.7.9, Minecraft 14w21b, Minecraft 1.7.10, Minecraft 14w30c, Minecraft 14w31a, Minecraft 1.8, Minecraft 1.8.1-pre1, Minecraft 1.8.1
Windows 7 SP1 64 bit, Java 1.7.0_51-b13 64 bit, nVidia GTX 760, Intel Core i7-860
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:
Here is an example with just moving the mouse in a small circle with the frame rate locked at 30 fps:
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.