Another Escort Tranny Trouble Tread

My transmission on my 94 1.9l escort goes from 1 to 3rd and no more. I replaced the speed sensor (for a new one) but no luck, funny thing is , than when it goes 1,2,3,4 speeds (happens rarely) its like a new car, it goes smothly and just perfect.
This problem happened suddenly, with like i said the rare occation it goes back to 4 speeds.