Wheel imbalance will come on gradually and get worse with speed. With the small tires on the focus, there's not enough mass to bring it on at lower speeds
Do you have steel or stock mags?
Don't look for terrible things to be wrong.
Look for bent or warped wheels. A dented wheel will balance, but wont track even on the road.
Make sure that the balancer centers on the center hole on the wheel and make sure it is clean. Don't use an adjustable lug hole balancer.
I had a Datsun 240Z that had the Datsun "jiggle" and all the balancing never
corrected it, until I used my friends bubble balancer
It could also be a dented steel belt in the tire.