Car begins to shake and vibrate intermittently above 50 mph.

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Car runs great until above 50 mph. You never know when it will start shaking. The shaking starts slowly and with a 5 to 10 seconds shakes and vibration escalate. When you slow below 20 mph or to a stop the vibration stops. Resuming highway the car runs fine til the next session which occurs unannounced. No apparent cause or reason.