Stuck in first gear?

Welcome to my world! It's easy to feel stupid AFTER you figure out the problem, but remember that most of the other people you asked didn't know either. This is how I learned best when fixing tvs, and unfortunately I learned a pile, ... The hard way.

The fact that it's a "cold only" problem tends to point to a sticking valve. I would have the fluid looked at and if it is NOT dark and burned, I would try having it flushed.

The torque converter is unlocking intermittently. The clue is your observation of the 200 rpm change which is exactly how much engine speed will change. The torque converter unlocks under heavy acceleration to create a doubling of torque, and it will unlock in what it thinks is a preparation for coming to a stop. Releasing the accelerator pedal to less than about 10 percent will initiate an unlock. Tapping the brake pedal will do that too, although that will also cancel the cruise control. You can duplicate the symptom by holding the accelerator perfectly steady while driving in third or fourth gear over about 45 miles per hour, then briefly tapping the brake pedal. You'll see engine speed go up 200 rpm, then drop back down one or two seconds later when the torque converter relocks.

When this problem starts occurring on its own, use your toes to hold up the brake pedal. If the problem stops, check the adjustment of the brake light switch.

In rare instances it is possible for the throttle position sensor to cause this but usually it's because the signal voltage pops to 0.0 or 5.0 volts. Both of those will be detected as unacceptable conditions, set a diagnostic fault code, and turn on the Check Engine light. If that's not happening, most likely the throttle position sensor isn't the cause.

You can also connect a scanner to view live data to see what is triggering the unlock. The status of the brake light switch will be shown along with various sensor data. By recording a few seconds of information when the unlock occurs, you can play it back slowly, frame-by-frame and see the data in more detail to see what's changing.

Replace the transmission fluid and filter if it continues bring it in and have it pressure tested-probable cause could be a shifting solenoid/valve body etc-

This sounds like the transmission needs to be serviced? Also lets run the codes to see what comes up these guides should help us:

https://www.2carpros.com/articles/how-to-service-an-automatic-transmission

and

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

Please run down these guides and report back.