I've had this shimmying for months now. The steering wheel starts to shake when I hit speeds over 70 kms. It comes and goes. Sometimes I'll press the brakes and it goes away. Other times it stops when I slow down or when the surface of the road changes.I thought that my wheels were unbalanced because they were bald and needed replacing. I've recently got 4 brand new balanced tires. The shaking has diminished only slightly. The ride is much smoother now but I still have this vibration. What could it be?