If you are reaching the rev cut out point and only shifts into 3rd by backing off on the gas pedal to release the load, the K1 clutches could be slipping or a leak in the K1 accumulator, valve Body. The K1 applies for 3rd gear.

Do you have a shift into 4th gear. An oil pressure gauge would aid in diagnosing this problem. You can install the K1 spring Kit MB # 126 270 44 77 it’s about $12, it’s a quick and easy installation to do with the transmission in the car.