Hello all I am in need of your advise. I have 2000 A6 4.2 Quattro Tip w/ 60,000 mi. w/ No codes

While drive at a constant speed of 45+ and I slow for a sharp turn or a stop light the engine will reduce in RPM's and the tranny will not down shift fast enough and cause the engine to die. If I use the TIP and manually down shift all works fine. When it does die it will start right back up with out hesitation. Also, today when I was driving on the freeway I put the car in neutral and the engine just died. I tried it a couple of times and it happed again both times. I am thinking it is the torque converter? Or ??????

Thanks
Jeff