DanB,
My guess is out-of-round tires, and possibly something in the front end causing uneven wear resulting in out-of-round tires. Find a shop with a Hunter GSP9700 balancer/analyzer and have your wheels and tires checked for balance, out-of-round, runout, force variation, etc. - all the things that the Hunter GSP9700 can find. See this
web page for a machine locator.
Good luck!
Scott
