i would check for corrosion on the battery terminals but your mechanic should have done that first thing. incorrect voltage can cause the problems you are running across. usually the ignition switch doesnt go bad it is the harness that goes to the back of the switch but when this happens the car will cut off as soon as it is started. its possible it could be starting to go bad. with the car running wiggle the key back and forth slightly and see if the dash lights come on (brake, alt, ect. not the illumination lights) or the car cuts off. if so its the harness.