I have a Shimano 3 x 9, XTR RD

When downshifting from the 3rd largest to the next smallest cog, the chain downshifts then rolls back up to the 3rd largest cog.
It will rotate on the 4th cog a time or 2, then roll back up to the 3rd cog.
Eventually, it will jump back down to the 4th cog, sometimes after rotating off chain tooth center, before shifting back down to the 4th cog.

This happens regardless of whether the the chain is on the small, middle, or largest front chainwheel.

Does anyone know the cause & cure for this problem?

Thanks In Advance,
Uni