2005 Montego FWD 110k

My grandfather stopped driving and gave me his Montego which had been mostly sitting for the past few years. It drives fine EXCEPT you can't force a downshift with anything more than gentle pressure on the skinny pedal. Don't try to pass someone!

​​​1) If you floor it from a stop, no problems shifting sequentially
2) if you are in motion and you floor it or go to heavy, chances are it will start BUCKING and/or THUMPING but it won't downshift.
3) if you are going up a grade it will shift as needed with no problems4) if you are very gentle on the pedal it will downshift and allow you to accelerate

Now I've put over ten thousand miles on the vehicle just driving it very gently, and as this car is an extra vehicle and I'll probably never get it fixed. But as a car guy who is always trying to learn I'm curious just what you think has gone wrong with the tranny. Fluid level and color seemed normal.