Wow! I really like both answers above -- both are worth checking out.
RE: the tires, if when you have rotated them, you switched backs and fronts, and you have a bad tire on the right side, you would still get the problem. You might want to put your spare in place of one of the right side tires, and if that fixes the problem you know Steve was right.
The thing I can't figure is why, if it is tires, it would get better for a while after servicing the car and 100 miles later go lame ...
My 2 cents re: tires -- when mine need replacing, I am going to go with Nokian NRVs. These received a great review in The STAR last year. There are a number of dealers around who can ship them to a local mechanic for mounting.