02 Cadillac SLS problems

cadillac
seville

#1
  1. Had a vibration in steering and seat at 65-70 with original Goodyear tires. Replaced tires, still have vibration, even after balancing new tires 3 more times. What could be the cause?

    2. Randomly I get a message in the dash to "Service Theft System, and the traction control light comes on. When I turn the car off, it resets and no lights or message. What is causing this?

    Thanks,