2001 Trooper Automatic Transmission Slippage

We have run into a daunting problem with a 2001 Isuzu Trooper with the 3.6. It came to the shop with a slipping transmission after it warmed up. We determined the transmission (80k miles) had an internal problem and the customer opted to replace the transmission with a used one. To make a long story short, 2 used transmissions later we still have slipping in 2nd gear after the transmission goes about 150 miles. I cannot believe it is another transmission, there are no defect codes. I am out of airspeed, altitude and ideas. Any Japanese car lovers out there with an idea?