I have reported video stutter issue many months back. For a different scenario but underlying cause may be similar:
(1) Anytime ABR is on or VP8 to H.264 transcoding is happening. Could also happen H.264 to VP8
(2) Real reason most likely is that VP8 input is a Variable Frame Rate (or Bit Rate or Reference frames coming in with long interval AND H.264 encoder can not track this input change.
VP8 video should be decoded and converted to 30FPS and then encoded to H.264 may eliminate jitter if what I am hypothesizing is true.
Did you test iPhone to Chrome video call? Look carefully and you may find the video Jitter.
Sent from my iPhone