On my recent 1100 mile trip, the rear end starting shaking me out of my seat at any speed over 60 mph. It would shake vigorously for exactly 30 seconds then drive smooth as silk for 30 seconds then repeat this cycle. I stopped anyway and had the tires balanced and rotated at a highly recommended Tesla capable store. No improvement. I was thinking perhaps software so I tried the power refresh but no help. I have an appointment at my SC tomorrow. Anyone else experienced this?