TRANSMISSON HELP!!!!!!!!!!!!!!!!!!!!!!!!!!…

I got a 2001 nissan maxima SE wit 89k and there wasnt no problem until just now when i was driving for about 30 minutes i was on a light and then i pressed the pedal the RPM went to 1 then quicky to 2 ,3,4,5,6 and the car was only going 20pmh and then the werid thing is it doesnt jerk it just doesnt shift i guess, then went to the specialist he ran the computer and it said outpout speed sonsor, then he reset it for me and i turned on and drived it diddnt do it until the next 30 minutes again???? is this transmisson??? is the sensor this important that it can do this?????? NOW THIS MORNING HE RAN THE COMPUTER AND THERE WAS NO ERROR AT ALL??????? HE SAID MAYBE DIRT GOT IN IT YESTURDAY?????? THATS WERID???? IS IT POSSIBLE?