Thumping Noise During High Speed Turns

[727LIG]

For some time now, I have experienced a loud thumping sound from the lower front part of my car when I am driving on a curved road at greater than 40 MPH. I live on a street that has a 45 MPH speed limit and several S-curves, so I hear this noise every day going to and from work. It appears louder when the car is cold. At first I thought it was the tires, because the noise was in synch with the tire rotation. I took the car to my Lexus dealer, and they said the problem was definitely bad tires. I went back to my tire dealer who replaced 3 of the 4 tires. The problem seemed to go away for a while. But it quickly came back, so I visited the tire dealer at least 6 different times with the same complaint. Each time, they checked each tire for balance and tread wear and judged them to be A-OK. They rotated them and even replaced 1 of them trying to solve the problem. Since I was having other problems with the tires (Goodyear Assurance ComfortTread) not holding air, I replaced them with a set of Michelin Primacys, hoping the noise would go away. It did not. Since my Lexus dealer is 120 miles away, I am reluctant to take it there until I feel I have a better handle on what the noise might be. I don't want them to dismiss the problem as being with the tires, when it clearly is not.

Anybody have any suggestions?