Confused

Hello everybody I am new to this site and would appreciate any help that anyone can give. I have a 1994 Concourse Deville that used to drive like a dream. My nightmare problem has been diagnosed wrong by 2 different mechanics. My car studders at stop lights and stop signs and it also takes off extreamly slow as if it is not getting enough gas. Another problem it has is late shifting. I was told that it was the tranny but AAMCO said it was fine then I was told that it needed a tune up but again they were wrong because I had a tune up recently..........please help me Thanks!!!!