1998 Audi A6 Can it be repaired?

A friend of my son drained the transmission fluid by mistake instead of the engine oil from his 1998 Audi A6 Quattro, after which my son drove the car with little or no transmission fluid for approximately 25 miles. After refilling the the transmission fluid it works fine but occassionally will not shift beyond second gear. He has to shut the car off and restart it to get it to work again. Can this problem be diagnosed and possibly repaired without replacing the entire transmission? Where should I start to try to determine if it is repairable? Thanks.