Originally Posted by Zaphod Beeblebrox

It was happening to a friend of mine over this past weekend. When she was pedaling at a constant cadence or accelerating there was no issue, but if she coasted and then resumed pedaling she'd get ghost shifts.

Problem turned out to be a shot RD. The joints in the parallelogram were badly worn out and at rest you could move the RD about 1.5 cogs space in either direction. Trashola. Swapped in a new RD and everything was perfect.