1989 Ford Escort transmission

It runs and shifts great, until it gets warm, then I hear the poppn noise like something has broke?Then I will have no gears period. After its cut off and cooled it will go and change gears without any problem-until it gets warm agin. It has been run extremely low on transmission fluid, we found when it broke down the first time, but since its had plenty, doesnt leak, and runs fine until it gets warmed up. Is there a saftey or relay switch on the tranny sumwhere