My 99 Escort SE sedan just shook real bad and shut off while in reverse yesterday. After starting it again it shook but then I put it in drive and started moving and it was fine. I know that the valve seats (I think thats what they are called) are going because the car has a clicking noise coming from the engine. So my question is, Is this shutting off from the valve seats or could it be something as simple as a tune up? I change the oil every 3000 miles but I have not yet done a tune up on the car. I have owned it since november of 2014. Any help is greatly appreciated!!