Will not shift out of third gear

First let me state that this is the second transmission in the car. The original transmission blew at approximately 140,000 miles, due to a similar issue, but ran until 160,000 miles when we replaced it with a transmission that had 60,000 miles on it. Now with approximately 130,000 to 140,000 miles, my transmission was running rough, finally clicked that she was not leaving third gear. A transmission flush helped prolong the last one, but this one is not showing any loss of fluid and color is still good. I do not have a check engine light on (shocking considering her mileage). I really do not want to buy another transmission for her. Does anyone know how to fix this? I need her to last another six plus months and I do have to shift gears up to three hundred times per day (I deliver newspapers).