Why does my steerting wheel vibrate when braking over 50 MPH?

I have an Infiniti I30 and my steering wheel vibrates when I apply brakes at sppeds over 50 MPH. Had brake pads, wheel bearings and rotors replaced at Midas in February. Took it back to them and they said that a caliper was sticking. Replaced both front calipers and pads again and the problem still exists. Braking feels normal at lower speeds. Do not wanna to go back to Midas because they obviously do not want to stand behind their work and can't seem to get it right. Could someone list the possible causes of this problem before I go to a different shop and get robbed again? At least I'll have some idea of what is ridiculous or not. Thanks.