seatomidwest

My AT transmission is failing to switch into 4th gear. It is more likely to fail the longer I have been driving (the hotter it is?). If I drive it a long way, it even starts having problems switching into third. I am basically ignorant of my duties as a car owner, and did not have the transmission fluid flushed for a long time. I bought the car 160k, and recently, at 260k, when it started having these problems, I had the transmission flushed. And suddenly everything was better! But not for long. It seems that the new tranny fluid only temporarily alleviated the problem. Am I just going to need a new transmission altogether or does the fact that new fluid made the problem temporarily go away act as any sort of clue that something lesser might be wrong?