Plane collisions seem to penetrate the plane and render a frame before the collision kicks in a lifts the particle back to the plane. This happens only with high gravity (>100) and relatively large radius (0.5) and when bounce and dampen is both zero. Oddly, it only happens in editor mode. When entering play mode it is fine.

Repro:
1) Open attached project
2) Open jitter.unity
3) in scene mode select the particle system
4) Observe jumping particle
5) play the scene
6) Observe no jumping