Ric - I had exactly the same problem with my original wheels from Japan (really nice AVS Model 5's) and it was always making loads of wobble at 70mph. Nothing before or after 70...
For me, it was all down to a buckled front wheel - as much as I've had wheel balancing done many times, a buckle which might not even be visually that obvious can still cause huge problems and no matter how much wheel balancing you do, you just can't get rid of it.
I'm hoping it's not but it's worth going over the wheels as much as you can and see if there's any sort of buckle or slight damage.
The UK potholes are the biggest culprits - arses!!! :wack: