Dave, I’d check the wheels very closely. I chased my tail a few months ago on a vibration that only started above 70 (not on the ‘51). Tire shop couldn’t find the cause. I had another mounted good tire/wheel so just started swapping it around, test, move test wheel, road test…..
After determining it was the left rear I carried the wheel/tire back to the shop and they finely agreed it was very slightly bent but assured me it was not enough to be a problem.
One new wheel later and problem solved.
Good Luck
RonR
