what is the fix for 1st gear slipping on 04 nissan quest se?

we have a 2004 nissan quest se, when de-accelerating down to under 20mph it thumps when going back into 1st gear, when attempting to re-accelerate it kicks/jumps out of 1st gear, also at times when pressing the gas pedal the van revs up but gives no acceleration, as if the van is in neutral, then all of a sudden it kicks into gear. all the fluids are topped off. it only does this in 1st gear. is there a solution to this issue or is this a guessing game with our pockets getting hit? could this be electrical?any help or guidance is greatly appreciated. the van has roughly 140000miles.
Thanks