transmission problem with 97 elantra

Someone has posted farther down about their transmission sticking. Mine does the same thing. I have a 97 Elantra. I did buy it used but not from a Hyundai dealer. It's got almost 79,000 miles on it. The problem started towards the end of the summer. I was going away to college and wouldn't be needing it, so I just drove it less for the rest of the summer. I tried changing the transmission fluid and that helped for a while, but the transmission started sticking again. I am getting it inspected on Friday and I was wondering if anyone knew what could be the problem and approx. how much it will cost to fix, so maybe if the cost isn't too high I can get it looked at. Thanks.

Meghan