Dave....Don't want to put the mokkers on things, but this is exactly what happened on my Sapphire and My Hockenheims. Had a vibration from the back of the car at about eighty mph. Took the car into a tyre place who had some modern balancing machines... They balanced all four wheels for me, and off I went down the motorway...... Exactly the same... Vibrating at about 80mph.. Took the car straight back to the tyre place and they took off all four wheels and put them on the balancer again to check them and they were perfectly balanced.

Then............. One of the guys looked under the cover that they place over the wheel while they were balancing them, and called me over. He showed me the run-out on the inner side of two of the rims....Both wheels were buckled on the inner rims..... You could see what appeared to be the wheels bobbing up and down... Thought I was F-cked, but someone recomended Ian Jemmison Engineering, and he "Somehow" got them perfectly round again, and now I have no vibration at all.

And I drive really carefully, never been kerbed, never been in a pot-hole, so I have no idea how it happened.

Also my buddy with his 4 x 4 Sapphire recently sold his Rondells, and when the guy came to buy them, (Obviously a guy in the know...) checked all the wheels and one of them was buckled on the inner rim...... They're a great looking rim, but like mine....made out of pig metal.

Would be well worth you getting the wheels spun up, and checking the inner side of the rim for any run out our buckling.

Hope it's not.............But.............