Its the Valve body in you're transmission. My friend had this problem before on his C280. His car would only start to reverse once he press the accelorator and would force the car back, and would force the car forward and then drive fine. There's was no auto matic rolling of the car once he let go of the brakes. They had a hard time figuring this problem out as his car still drove fine and everyone first doubted the tranny. It was once they finally took the transmission apart(cert. shop) they discovered this problem. It cost him around $2000 dollars to have it corrected, much cheaper then a new transmission.