Harry8160

After driving for 60-90 minutes a vibration begins in the rear of car (feels like a flat tire) and slowly moves to front where it gets so bad I have to stop the car. When applying brakes there is excessive front wheel shimmering. There is nothing wrong with the tires after stopping. When I resume driving the shimmer is gone and does not starrt again for another hour. I had the wheels balanced, alignment checked and brakes checked. All OK. Any suggestions as to cause or solution. Thanks