Erratic Shifting.

The trans throttle valve pressure cable or detent or kickdown cable as it is commonly called can cause odd shifting.
It is on the braket that opend the throttle plenum butterfly at the throat of the engine...There is a small platic grommet /spacer that commonly breaks and falls off.
This allow the cable to have about 1/4-3/8 inch more slack in it ans softedn shifts...lowers shift points and causes a lot of slippage and wear to tranny...
THis is an easy check/fix/
Yours sounds more like a clogged up fliter in the pan though.
I'd try a pan drop and filter/fluid change.
BTW you can drian the torque converter too with the rubber plug in the bottow of the bell housing..
You have to turn the engine over by hand (balancer front bolt or through the hole mentioned above) until you can get to the little pipe plug that drains it.
I think it is an 11 mm hex socket to remove it.
It will hold almost 5 with just an drop and almost 12 with converter drain also.