95 lumina ls 3.4l 88k miles transmission over heat help

Ok so i have had some issues when the trans warms up after fast of highway driving or even sometimes sitting idle.. I get a check engine light and lose all power in first almost feels likestarting in 2nd or 3rd. Car feels like a overloaded boat slowly accelerating at 3-4k rpms , after about 25mph it doses a down/up shift then seems to run ok.
Recently I had a oil change done and they added trans fluid apon inspection. I Think they may have overfilled it because the car was fine before. So recently I changed the trans pan gasket and filter due to a small leak in gasket.( which they pointed out)
When the car is cold or hasn't run over 50mph its fine shifts smoothly through all gears, It There something I can try or check next ?, I know that there is the check engine light code in ther but I bought a reader and get nothing from the obd1 plug. Any one know where I could gt a reader thta would work? Dealer wants way to much and will try to slam me onother stuff (airbag Light)
Thanks in advance for Anything

You need the codes and/or have the trans scanned specifically, with a trans function scanner.