I don't think there's sufficient mass in the rings to cause the planet to slow or accelerate via tidal drag.

I have been wondering if there might be a cyclic variation related to its orbit or its relationship with Jupiter, but I can think of no mechanism that would cause it. I think many more measurements are going to have to be taken, probably over many decades, to establish if it is a periodic variation. Maybe some other testable mechanism will be suggested.

However, I'm just blowing off blatant speculation here -- I don't really have a clue at all!

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Stephen