2.2 Shifting Problems

'91 Corsica 2.2l with 278,000 miles - GREAT car! I am finding it very hard to give up on her - Suddenly, while going uphill car will not downshift normally. When I floored the accelerator the car downshifted but then would not upshift anymore. Now will only shift from 1 to 2 at 40 mph and into 3 at only 70 mph roughly. New fluid (with Lucas) and filter, car ran good once after this treatment. I noticed that when it did shift properly, shifting from neutral to drive was very smooth. When it doesn't shift properly from neutral to drive the engagement and engine are very rough. Repair shop said transmission is shot, I disagree because the car did run properly afterward. It seems that transmission malfunction coincides with slightly poor engine performance and engagement of neutral to drive. I would greatly appreciate your input!