Transmission hesitates when warm
 

Hello all. My questions concerns a 2002 hyundai elantra that a friend of mine owns. Later he seems to be having transmission issues. WHen the tranny is cold it has no problems but however after driving for about 20 minutes he seems to be getting hesitation/sticking between 2nd and 3rd. Says it is usually between 20 - 40 mph. When car is past 40 there are no issues at all. Car has 103,000 miles. Had it to a dealer and the code read that he had a bad oxygen sensor. Had the sensor changed and the transmission fluid changed but is still having the same issues. What are some of the things that may be causing this issue. He hopes it is not in need of some major work. Any hints would help when he visits the dealer again so he knows what they might say and charge. Thank you.

same problem with mine...

when i get up to 30mph the car jerks forward like it downshifted and feels like it lost power intill i get to 40+. i noticed when i hit 30mph the rpms wont go past 2000 enless i slam on the gas pedal.

It feels as if i am running in 2nd gear going 40mph and the engine is braking. like 3rd gear does not engage.
been happening for 2 years....