my 1995 960 has an intermitent problem-when i reverse out of a parking space while turning the wheel counterclockwise, i feel like i am rolling over a bump when the ground is clear. in the space it takes to get out of the parking space, i will feel the bump about 2-3 times. sometimes it feels larger than others. it doesn't happen every day and i haven't been able to attribute the cause to anything. tires are inflated properly (though i thought the tires might have a flat spot in them), brakes are good. i had mechanic check it out recently -he felt the problem but couldn't find a cause. please help!