i own a nissan maxima 2005 and i am having trouble when it changes to second gear?

[JuancarlosCoria]

i own a nissan maxima 2005 and i am having problems when i drive for a long time in the city the transmission puts in second gear hard and sometimes it loses the gear i think it might be a problem with one tof the sensors. i would like to know what sensor could be causing this problem?