98 lumina transmission question

hi I have a 98 lumina with a 3.1 it runs great however recently i noticed that when I travel at around 50 mph up a steep hill (semi highway) the car does not want to shift all the way. with the gas steady it seems to go back and forth between a high gear and a lower. if i give it alot more gas it does shift all the way into a high gear but with a 40 mph spped limit , cant really go above 50. It shifts very well on a more level road.

any ideas?? it has 110,000 mile and purchased 6 months ago - not sure if the transmission fluid has ever been changed. the fluid is pink to brown

Does this sound like :
a fluid change issue or something else
would a transmission additive help

thanks!!

would you call what your vehicle is doing "jerking"? Lots of people on here have that problem, there doesn't seem to be a "universal solution"

yes i would call this jerking - someone thinks it might be a fuel filter others say it is the transmission

yes i would call this jerking - someone thinks it might be a fuel filter others say it is the transmission
Changing the fuel filter is easy/cheap enough, but bad plugs, wires can do this, a slipping torque convertor lock up can do this, a bad tps can do this, should have a full scan and look at misfiire counts and/or trans codes.