The transmission in my '98 Cavalier Z-24 contantly shifts randomly between 2nd, 3rd, & 4th gear while driving btween 25 & 40 mph. It's fine at higher speeds, and when the speed is below 20 mph. The trans isn't slipping and the engine is running fine. Is there anything I should look for that might be causing this to happen, such as a broken vacuum line, before spending big cash to have it fixed?