2004 Kia driving and it hesitates

Engine problem
2004 Kia V6 Front Wheel Drive Automatic 88000 miles

Original owner of a 2004 Kia Amanti, did not see it on the list. I take it regualry to the dealership till Oct when I found out (the hard way) they were billing but not not always doing all the work.

A couple of weeks ago, air filter changed and replaced head-light bulbs.

I have a list from the dealership said the car needs a transmission flush and fuel system service. I don't really know what this means. They said the mechanic is too busy to tell me, just bring it in.

Anyway, I am scheduling an oil change in this week but the car has a problem I don't know how to explian it well. When driving (not on cold start) it is fine to 35 mph. At times btw 40-50 it hesitates. Once above 50 it is fine again. I call it hesitates, my old car was manual, this is not like it is reving and not changing gears. This is more to me like it is hesitating to accelerate. Will the flush and or fuel system fix this?
Thanks! I do appreciate the help. The dealership is so expensive, they said this work will take at least 4 hr and thy always take at least 2 hr longer than they say so I need to make sure I can get this fixed and timely. I want to take it else where but worry it is a "dealership" only repair.
Thanks!