4L65E Only works when cold 200-300'

[k72nova]

Short Question: What would cause a 4L65e to only work for a few hundred feet before giving out, not throwing any codes

Longer Story: I’m starting a new project which has the foundation of a 2001 Yukon Denali. I bought the Yukon cheap because the transmission doesn’t work. Well I go pick it up and drive it right onto the trailer, yay! Except when I’m pulling it off the trailer, I get about 120’ and the trans stops working. Come back a couple days later and move the truck maybe another 200-300’ and before giving up, again the next day. Anyway I bought the truck with the intention of buying a junkyard 4L65e and swapping it but I’m starting to think if I can just fix the one that’s in it I would rather go that route since I’m having a hard time finding anything but a 4L60e. The guy I bought it from tells me it’s the second transmission this has had in it and it was replaced roughly 40k miles ago. They tried putting a new trans cooler in it thinking that was the issue but that didn’t fix the problem. The trans is full of fluid, that’s the first thing I checked. I’m wondering what could cause it to only want to move when it’s cold? I’ve tried running the trans through the gears and they’re all neutral after a few hundred feet.