So, I have a 2005 Pontiac Grand Prix with a 3.8L and a 4T65E Transmission.
It has roughly around 200k miles.

Since I bought this car a few years ago, the transmission "surges" under light engine load.
By this I mean, if I have the accelerator at 10%, it'* fine. If I have the accelerator at 20% it surges. If I have the accelerator at 30% it'* fine.

At first I thought it was the electronic throttle actuator or accelerator pedal. I replaced the throttle assembly with an OEM GM part. (brand new)
Still happened, so I was going to replace the accelerator until I tried resetting the Trans Adapts. Problem went away.

I am ASE certified, but not in transmissions and I never rebuilt any transmissions either. I can usually diagnose them, but this one is beyond my knowledge.
All that I've done to this car is... I pulled the engine and completely re-gasketed it with a few new parts since the rear main was leaking badly.
I did a drain and fill/filter on the transmission and replaced the input shaft seal.

Watching the scanner while driving, I notice that slip speed is normal and there doesn't appear to be any problems with the torque converter itself.
The Adapts are high and I get codes for P1811 Maximum adapt and long shift, P0703 Brake switch circuit 2 (sometimes) and I have the occasional problem with a U1016 Loss of communications that kills my instrument cluster and turns off the A/C compressor during the summer while I'm driving.

Like I said, it'* pretty intermittent and if I clear the Trans Adapts it goes away completely until the transmission is finished with the relearn.

I'm planning on either rebuilding or replacing this transmission in the very near future, but I'd like to know beforehand what the likely cause is since this is something I've been slowly trying to diagnose myself.

Any help from actual transmission experts or someone that has had this problem before and solved it would be great.

When was the last tuneup?

Roughly around 20k miles ago when the engine was pulled.
This was happening before the engine being pulled and even after it was reinstalled.
I've been a mechanic for well over 15 years and have all the diagnostic equipment this car would ever need including a scanner and programmer.

What kind of programmer?

Autel MF2534

I hate intermittent issues.

We had one here a few weeks ago that had TCC unlock/lock issues because he was at the TPS threshold.

Any way you can tell if EGR is opening during this?

I watched the EGR on the scanner and it'* working like it should. However, as I've said... The problem goes away when I reset the trans adapts. When the transmission learns to stop slipping gears, it starts happening again.

Did all of this start happening AFTER the fluid/filter/pan drop?

Before. Pretty much was happening when I bought it.

Do you remember what the fluid looked like when you dropped the pan?

How'* the fluid now? Smell burnt, brownish?