My car hesitates on pull off but runs just fine once it is going? 2003 Diamante.. i used fuel system cleaner?

i drove the car in ths shape over 700 miles on the highwayover the mountains so i am pretty sure its not the transmission . new battery, alternator, radiator, brakes and tires. my car sat for about 3 months not driven. theres a point of tension at which the car operates great but at anytime if u punch the gas the car will decelerate all fluids are good.. i just need some ideas now as to where to start with anything that i can do myself before i take my car to a mechanic. please help