joe_d
Everything in the transmission is OK. We have confirmed that the problem is due to overheating. When the truck cools down, transmission works fine. The exhaust tube that goes to the muffler passes to close to the part of the radiator where the trans. oil cooler is located. Actually it is the transmission oil that heats up so much that it is overheating the truck's coolant. Have complained to Clark and they only respond to me that they've never seen this problem!
Thanks for the Royal Purple tip. I'll try this coolant on this truck after I modify what I can of the exhaust pipes.
Cheers!