Update:
Turns out the issue was the ipr valve. My dads mechanic replaced it before he had the chance to run the truck with the scan tool. Also learned the ipr valve did initially throw a code, once, which his mechanic failed to tell him. After that it never coded again. My dad said the spring on the valve was very worn out probably causing the valve to either close/remain open once it got hot, truck shut off then cooled off and cycle repeated.(at least that's my theory). My dad says thank you very much in all this, also learned his mechanic has turned into more of a parts replacer instead of a troubleshooter and will likely be looking for a different shop. He just ran the truck for 3.5 solid hours at 65mph for almost 200 miles pulling 20k lbs of hay without difficulty.Thanks again!