2003 BMW X5 serious shaking and vibrating

2003 BMW X5 3.0 liter mileage: 61,000. Experienced serious shaking and vibrating with the entire car; happened for a few minutes, then went away. Next time I'd drive, it would be fine for ten minutes then the vibration begins again. Escalated to a point where I thought the engine would drop to the road because the vibration was so bad. Took it to dealership; "firing" problem, leak, or so the computer diagnosis said. They replaced spark plugs, gasket cover and thermostat to a tune of over $700 dollars. Drove away and within ten minutes it shakes and vibrates, as bad or worse then before. What could it be now? Obviously, it wasn't the plugs and gasket cover!