Sorry again for the delay. This appears to be stuttering that happens when the game FPS is slightly different from the physics FPS. It depends on the particular interaction of those two frequencies. It's common for the game update rate to vary slightly, be capped at vsync, and it's possible for it to dip a large amount, for example when another app suddenly takes a lot of resources. Physics accumulates time based on the game update rate, but physics only updates at its configured rate, with any remaining time spilling over to the next game update. For these reasons it's difficult to control the interactions.
Increasing the physics update rate generally makes the stuttering smaller, but isn't a real solution, again depending on the interactions of the update rates of the game and physics.
The easiest solution is to remove the fixed time step from physics. When physics runs at the same update rate as the game, stuttering won't occur. The downside is that physics behavior is dependent on the update rate. If the game FPS varies significantly from the update rate the physics settings were designed for, the behavior may be undesirable. For example, things may not move as fast or as far, or they may move too fast or too far. This can be mitigated by capping the physics update rate. For example, if the target game update rate is 60 FPS, allow physics to update between 50 and 70 FPS. Larger variations of the game update rate would cause stuttering.
Another solution is to interpolate between the last 2 physics states. With this approach, when physics is updated, the difference is not applied right away. Instead every game frame the amount of that difference that is used is based on this percentage: the time elapsed since the update divided by the time between updates. This interpolation ensures smooth movement between physics updates, regardless of alignment of the game and physics update rates. The downside is it's a little more work to remember and apply the values, and especially that then the physics pose is always behind from between 0% to 100% of the physics update rate. For example, at 60 FPS physics would have average lag of 1 / 60 * 50% = 8.3ms with worst case lag of 1 / 60 = 16.6ms.
Unfortunately, fixing this exceeds the complexity of a change we're comfortable with making to a stable version (4.2). Of the two solutions interpolation is better, so we would like to implement that in 4.3. If you need assistance having physics run at the game update rate, we can help with that.
