I had this same problem on an '86 Ferd gasser. The trans fluid went from the tranny to the radiator to the air cooler in front of the radiator then back to the tranny. I flushed the coolant twice, replaced the thermostat twice, even bought a huge after market radiator. The sum of all of that was it would cool back down really fast, but the fact remained that it still overheated. After I traded the truck in I found out it was the tranny all along. BTW I was only towing about 6000 lb camper at the time. My vote is with Triton, have the trans and torque converter checked out.