2000 Buick Century car troubles

I have a 2000 model buick century that has been in the shop over 5 times in the past year. Transmission was replaced 1 week after I bought it 3 years ago. Now it is sputtering especially at around 50-70 mph like it is not getting gas but the tank is full. It does not seem to have any get up and go anymore. I need this car to last me about another year. The mechanic mentioned the Cadallic converter (expensive to fix) but my parents have the same care and just replaced the cadallic converter and the problem was fixed for about a month now it is doing it again. Any ideas before I go drop hundreds repairing it again? (Also under driver side door it makes a ting. Ting. Ting. When the car is turned off, didn't know if that would help with anything, I have gotten mixed answers on that one.)
Thanks guys!