2005 focus zx4 120,000 miles. At speeds of 55-65,there is a shimmy/shake in the steering wheel. At 70 and over it is hardly noticeable. Would this be caused by a tire out of balance or a worn suspension part? Suspension is stock and has never been replaced that I know of,have had the car since 65,000 miles. Around 10,000 miles on the tires. thanks!!