How to reproduce:

1. Open attached project (reproProj.zip)
2. Open scene Scenes/Test.unity
3. Play the scene
- The cube should begin floating up and down
4. Open the Animator window
5. Go into the Blend Tree layer
6. Set the VelX and VelY properties to 0.5
- Note that the spinning skips frames and resets at regular intervals
- Setting the VelX and VelY properties to 1 causes the cube to spin diagonally (as expected), but not at a constant rate
- Reproduced in Version 5.4.0b6 (2c3d6b93e789), Version 5.3.2f1 (e87ab445ead0)
- Blend Trees are broken in 5.2.4f1

- By Design: This behaviour is an artefact of how Euler angles are blended.