im guessing it has an electronic gearbox ecu if its triptronic.

Have seen a few of these ecus fail on various models with triptronic - mainly audis but also a couple of vauxhalls too.

Worth checking first tho as they can be expensive.

A friend of mine has an A4 1.8T with triptonic and the gearbox would go into sport mode and back into drive all on its own - that is the gearbox ecu fucked and is a grand for a new one