ml270 cdi?

i have a ml 270 cdi on a 51 plate. to be honest the car has been faultless up to now. when driving along the car seems to get stuck into one gear and the tip tronic does not even work, and refuses to select any other gear. if i stop and restart the engine this seems to cure the problem and this seems to happen every now and again. any answers thanks/

mark
April 2009