My trans slips sometimes and wont shift to the next gear?

yesterday i was driving my automatic 1994 j30t infiniti and my car wouldnt go to second gear....it just kept raising the rpm so i pulled over to the side and turned my car off and then back on and drove off and it started switching gears normally what can this be from? my car has 189,000 miles on it and the trans mission fluid is full but mechanic said its dirty but i dont wanna flush or change it because im not sure when the last change was done...