Occasionally, the gears do not seem to engage. Instead of the normal muted clunk when going into "R" or "D" followed by a slight "creep" when releasing the brakes there is no forward or reverse movement at all despite trying all the options. Only when revving fairly fiercely to around 1200 to 1500 rpm does the car begin to move. Never had this before with previous mercs although this is the first five speed I have had. Any ideas/comments would be appreciated. Kent.