drain the oil out.look at the strainer.i'll bet the oil is black and the strainers full of bits.sounds like clutch packs shot but why?check for pack pressures, check for cooling circit pressure,check charge pump pressure,torque convertor pressure.usual stuff.
recently had very similar problem on a h7.00xl.the customer had a radiator damaged and decided to intall his own non hyster rad from god knows where.problem was the flow rate through the tranmission cooler was much less on the new rad than the old one.result was that the oil did boil and you can guess the rest.