I would take it back to the shop as it sounds like something wasn't hooked back properly. Anything from a Throttle Position Sensor failing to a simple Vaccum Line coming loose can cause this hesitation at speed. Also, the simple code reading done at the auto parts stores can be misleading. Sometimes a problem in between two sensors will log a code showing the two sensors to be bad or failing when in fact it's because something else is changing the dynamics of the cars performance........ie: Bad Gasoline, clogged injector, etc.
If you want too, raise the hood with the motor running and listen for a "sucking sound" as if air is being pulled in to a hose. Try it both at idle and with someone running the rpm up at bit. If you find a hose loose and sucking air, try and find where it should attach and you may can fix it without going back to the shop.
Finally, if the Fuel filter is starting to stop up and needs replaced it will indicate this need first at the higher RPM's because it can't filter the higher flow of gas. If you ignore it, finally it will start to stall and eventually leave you sitting.
I hope some of these ideas might help.
Good Luck,
Pinz