So an issue has popped up with my 06 GT over the past few weeks. Whenever I start my car (after sitting overnight or all day at work) and I put it in reverse, my car gets this odd periodic lurch or shake. It's better described as a muscle spasm - just a slight periodic shake or spasm. It even does it if I sit there with my foot on the brake and the car's not moving. It only does it when the engine is cold, and as soon as I put it in drive it stops. Any thoughts on what it could be?