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.

logic is sound.

tom w

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

your flex disc, I would look at the driveshaft support bearing as well

__________________
1988 300TE
1979 240D
1979 300SD(R.I.P. Hit by somone running a red light)