'90 Dakota AT won't pull when engine's warm

It's great when it's first started up, but once it runs 15 to 20 minutes it stops shifting gears, acts like no trans fluid is in it. But there is plenty fluid. Someone told me it needs a trans. someone else told me it needs the filters changed. someone else said that there is a sensor that goes out on that truck often... that gives the transmission the fluid when it needs it... Is this true? Now, today I just went to get it inspected and it failed because of high nox. If I fix transmission problem, will high nox problem be fixed too? I hope i hope i hope...........
Does anyone know ?

huh?

Sounds like a torque converter or the pump. A pressure check will confirm or eliminate the pump. You didn't mention which tranny or mileage, it could be that it is wore out. Pressure test will tell this also.