05 elantra.I dont know what the problem is, help? on 2005 Hyundai Elantra

I have a 2005 hyundai elantra, 108k miles on it and when I drive at constant throttle and imbetween around 30mph - 40mph my tachometer bounces up and down a bit, sometimes more. It seems to lose power sometimes to and the engine does rev in connection to the bouncing of the tach. Also sometimes when slowing down and I slow to about 20mph I get a jerk as if it went to too low of gear. I have rplaced TPS and the input speed sensor, do I need the output as well? Or is it even a speed sensor? Does anyone have an idea on this? There is no check engine light on.

Asked by for the 2005 Hyundai Elantra
Have the TPS and it's circuit tested directly using a real scanner and dvom to see if both readings match. Possible ECM or wiring problem. TPS signal has major influence on engine rpm and trans. shift.
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2 more answers , 2 more comments
agree with answer #1.might be a diagnostic job for dans auto in Lowellville.i used to live there on liberty st.
Dans auto did run a diagnostics on it, but got nothing and was nice and didnt charge me because of it.
It is possible you have an internal transmission problem. What does the fluid look like? it should be a clean pink color. Smell it too, does it smell burnt? If your transmission hasn't been serviced in a while I would start there. Expect to pay $100-$200 for the service.
It looks good and was done at around 88k miles. It doesnt smell burt either.