I recently had the front brake pads changed and the rotors machined on my 87 325. I put about 1200 interstate miles on it aftrerwards in about 2 days. Now when I am braking it a bit hard on speed the steering wheel begins to vibrate left and right . When I brake slowly at low speed everything is OK, just the brakes squeeal (it might be normal though). Does anybody know what can be the cause???