My car shifts timely on 1st and 2nd gear but it will shift late to 3rd gear and 4th gear. What its the problem

I have a 96 pontiac bonneville and before i put the new computer in my transmission shifted good till i put a new computer in and it was programmed and all and my transmission started shifting late on 3rd and 4th gears like it shifts to 3rd when i go 45 and it will go into 4th gear when i go 55 mph and it wont shift to 3rd gear when i dont have overdrive on. What is the problem?