Something is not right with the steering on my '05 C230. When stopped, if you turn the steering wheel off-center, you can feel a vibration/shudder through the wheel. At speed, the steering feels really vague, unplanted and light. If you are going around a turn at speed, like a highway ramp, and hit a bump, you really have to watch it because the steering feels so loose. It feels like the power steering is over-boosted. Power steering fluid level is good, and there isn't any noise when turning. I'm not sure what to do. Any thoughts?