Failures with the 4T40E automatic box

A very cordial greeting to all., Buy a Cavalier 2.2 of 95, with the good engine, does not smoke, accelerates without giving problems, and its march minimally is stable and very even, my doubt is because the lack of strength when applying the direct or the reverse, the car accelerates very slow How to discard until you find the solution, for this I require your experience. Thank you very much.-