it might sound daft but are your wheel nuts tight, and do they hold the wheels tight against the hub?
It sounds stupid but i changed my wheels, i was convinced they were done up and started to get a vibration above 70. Everything was checked and checked again until i found the threads in my hubs were rusty and the new bolts had not seated the wheel correctly even though everything was tight.
20 Years an Engineer, and it stumped me !