OK..understand. Easy causes are low fluid level, clogged transmission filter....then I would try to determine condition of the transmission modulator
: when throttle is down and vacuum goes to near atmospheric pressure, the modulator must increase oil pressure in transmission to keep clutches from slipping: a dead modulator would maintain low pressure and induce slippage when engine torque is high. THis is the last external component but after that, everything is possible....I would not like it either.