I was driving on the highway today, and I noticed that at around 55-60 mph, the car feels a little shaky -- almost like it's difficult to keep it going in a straight line. I feel a slight wobble in the steering wheel, and even with two hands on the wheel, I felt like I had a hard time keeping the car going straight.

What issues could cause something like this?

On a somewhat related question, would lowering the car (my car has stock suspension) make a stable car unstable or improve stability?