Does it ever do this while still cold (warming up) under the same circumstances?

The fluid is burnt and during the summer the trans fluid temp gets up between 200-240 degrees depending on the outside temperature and my driving habits.
Also, I know the clutches are starting to thin out more and more every day which is the reason I'm either rebuilding or replacing the transmission.
For the most part, I've never had this problem with any other transmission in any other car I've owned and I've almost always owned a GM.

Another thing is that it also mostly happens around 3rd and 4th gear. Possibly an overdrive band?

The engine runs at higher rpms until the transmission gets to 70 degrees or above. When the fluid temp is colder than 70 and it'* still in "warm-up" mode it doesn't do it.

It sounds like the converter clutch is slipping to me.

Does the TCC solenoid control pressure to the converter clutch? Or is it just an on/off type of solenoid?

It'* PWM.

Here check this thread (below) from this wicked cool forum member and see if it sounds familiar. Your transaxle is a not-so-distant relative of this cool guy'* transaxle and this issue (as a factory defect) was fixed within a year or two, but the same mechanisms exist to cause similar issues were a transaxle like yours to find itself in just the right circumstances.

https://www.gmforum.com/buick-172/19...urging-310146/

I thought about that as well and also installed an aftermarket Trans Cooler and completely bypassed the radiator cooling. With the worn clutches however and possibly a worn converter, the temps still get fairly high even after I changed the fluid and filter.
I'm guessing that I'll just buy a reman, but keep the original trans so I can tear it down and find the exact problem myself. I watched numerous youtube vids on this trans being torn down, diagnosed and rebuilt and I also have Mitchell, so when I eventually get into it this spring I'll add in what I find.

If the trans is getting that hot, its slipping, or the TCC is slipping.

These trans generally don't get too far beyond 200K before they show signs of needing a rebuild. Most people make the mistake of doing a pan/filter drop. This removes most of the suspended clutch material.

Time to rebuild.

Yep, if it'* slipping, especially so much that the auxiliary cooler can't keep up, it'* time for a rebuild.

So I was driving my car today and found that my Cruise Control wasn't working.
When I got home I hooked up the scanner and found that I have a few codes I didn't see before.
P0573 - Cruise brake switch circuit high voltage
U1016 - Lost communications with ecm/pcm/vehicle control module
U1301 - Class 2 data link high

I knew about that loss of communications code since that was there when I bought it and is mostly intermittent, but then again I always figured it was part of me installing an aftermarket radio and using a PAC RP5-GM11 Radio Replacement Interface which connects to the vehicles data system to tell all the modules that the radio is ok.
However, after researching the Cruise Brake Switch code I found that while it'* malfunctioning it can screw up the TCC signal causing the clutch to slip more and make the transmission fluid run hot.

I won't have time to check data lines and modules until the middle of next week, but this is something that I'll have to dig into before throwing a new transmission into it so that the same communication problems don't kill the new trans....
And since I'm a nice guy and I know there are a lot of people with the same U1000-U1016 issues, I'll let y'all know what I find. If I find anything since it'* so intermittent. I do plan on just replacing the brake switch though since it'* a cheap part.