BMW 520 ES wheel balance problem?

I recently got my two front tyres (continentals) changed and after the tyre depot balanced them I noticed a wheel balance problem.(steering wheel vibration) Took them back for rebalance but no better. The problem was between 50-65 MPH. I then got them balanced at a more specialist place and the chap said they were approx 25 gms out but that now the wheels were running very true. Unfortunately although a bit better I still have a problem around the 65-75 MPH region. Please help !