My 500 SEC has a vibration that seems to start around 65 mph. It is not constant but it seems to come in cycles that start, die down and start again.

I'm guessing that it would be the universal joint or joints. Maybe the flexplates? I've pretty much ruled out the tires being out of balance because (i think) that would cause a constant vibration. The tires are slightly oversize and I would like to return to the factory spec tires at some point.
Any ideas would be appreciated.