Please help!shifting hard and no power when hitting gas pedal between 25 and 40

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When im driving and get to about 25 mph it acts like it dont want to go any more..i have to ease on gas pedal slowly getting it to go and shift to next gear..it also has been shuttering when shifting and when im pressing on gas pedal. after i get to 40 its fine.. it goes and will keep going fine...stop at a stoplight and repeat again..replaced map sensor,fule filter,alternator,battery within the last 3 months all fluid and oil in car is fine. I have a 1 year old and im completey lost and tired of buying things that haven't worked. Please help! Thanks! :)