Steering Wheel problems of the 1996 Chevrolet Corsica

This vehicle has a front end steering vibration, at times severe, while driving straight and increases in a curve. The dealership has been contacted an they advised me it was either tire blance, out of round tires, or alignment. I bought new tires with balancing and alignment. This helped a little, but the vibration is still present. How can I get this taken care of?.