I have the same exact issue. Runs ok when cold, but horrible when it heats up

Hi, here is the response from Transtar: There will be residual fluid left in the transmission and torque converter during a “ pan removal service”. In the case of a U660E you would be able to extract approximately 3-4 +/- quarts of fluid depending how long the pan and filter was left off. This total is about half of what the transmission and torque converter utilize when completely full. As for whether there is a way or a hole for the old fluid flow out when transmission fluid change: There is no way to remove all the fluid form the transmission and torque converter when performing a “pan removal service” on a U660E transmission. The torque converter does not utilize a drain plug. There are Transmission flush services offered by many repair/service shops that will remove close to 100% of the existing fluid with new ATF. A specific machine is required for this service.

Got to 2:27 of the video

First CB, I said damn it how'd I do that? Must've walked away or answered the phone 2nd one a week later. No way I did that twice. Started digging and found NUMEROUS shops across the country 3rd was a few months later wasn't mine but still cost the shop. Supposedly had to load in press, torque down with red loctite. Leave it pressed for a bit, come back and give 10 more inch lbs. Jump through I million hoops for a part that never goes bad from factory, only aftermarket. Rumor is that gasket quietly disappeared