I had the same experience one time when it was a cool day on the highway at about 65 mph. Very smooth road just like you say. Since then I have not had the problem. Could it have been tire pressure?? Not a clue. But hasn't been a problem since. I have a '98 standard suspension with 540 wheels and pilot tires, what a combination! I love this car, and I have had some nice iron in my 37 years of driving!