1983 Toyota Tercel my transmission sticks

Hi there something wrong with my transmission this does not happen all the time on any random day I have a drive to work o back home my car feel like it cant reach higher speeds after it shifts to third gear it kinda gets stuck between 80 and 85 kmh but thats happens randomly I dont have have a pattern or at least i havent seen one some days i can get her to run 120kmh smoothly with no problems. one day, and I know this is probably the dumbest thing to do, she started to give problems in the transmission so I kinda forced her back by kicking in the throttle controlled overdrive she began stoping but at the end she came back up again and ran like a dream (im hoping i didnt mess her up)