This tranny has been losing fluid for the past couple of months, a mystery since there is no huge leak underneath. But it has steadily increased tranny fluid use until now it takes 1 quart a week to top it up. When it has proper fluid level it runs great, pulls hills fine (as much as a 2.5l non-turbo diesel can)

Last evening it shifted 1-2, 2-3 and then at about 45 mph the tach zoomed to 5000 rpm and it felt like it had no power beyond that, if you come to a stop and try again it does the same, pulls great until 45 mph then its like it hit a wall and slips and flares and reads 5000 rpm on the tach.
whats going on ? Is it the torque converter or the tranny itself ?