UIProgressBar.cs > line 115 (public float value) > in the setter, try changing the float value in
if (Mathf.Abs(before - this.value) > 0.001f)
to a value that's lower, like 0.0001f
I'm not sure how this will affect other NGUI components or even if it's a good fix, but it fixed my jerky slider issue.
Basically this happens when the change in value is too small to warrant an update. The slider waits until the delta is big enough and then jerks to the new position.