I think your idea that it's the trans is correct. Based on experience with my 1991 model (302 engine & 4 speed auto, probably same as yours) which also had low mileage at the time, the trans needs rebuilding. Find a good shop that can upgrade the trans instead of just putting it back to original. I had this done about 6 years ago and it's still strong - better than new - with about 150,000 on it.
I also changed the exhaust to duals when the original "Y" pipe rusted out. Car now runs slightly faster and quieter especially during high speed (50-80+) passing. | 
