Hi, I have read through everyone's responses here and most seem to think this is something to do with the tires mainly, plus other theories. Which may be true, but I recently had this very experience and tried all these remedies to no avail.
The problem actually was the Power Steering Pump, which finally just failed and it had to be replaced. As soon as this was complete all these issues you have described disappeared.
Possibly this would be a good place to have a look at next if your issue is not resolved.
