99 Corolla question

I recently bought a 99 corolla ce w/5 spd tranny. Since I've been driving it I've noticed that my gas mileage has been terrible, at best I'm prob getting around 20 mpg. The car seems to run great except when I pull out onto a road and give it a little harder push. If I have to pull out somewhere and go fast it seems to hesitate. I can push my foot down to the floor and the car almost starts to sputter as if it's not getting enough fuel or air. I took it back to the dealership and told them to look at it and they swear that there's nothing wrong. Does anyone have any ideas what might cause this?

Wires, plugs, 02 sensors, general tune up stuff maybe...

Am I correct in assuming that something like fouled fuel injectors or a bad EGR valve would show up on a diagnostic? That is assuming that the dealership isn't lying to me in the first place.

Am I correct in assuming that something like fouled fuel injectors or a bad EGR valve would show up on a diagnostic? That is assuming that the dealership isn't lying to me in the first place.

The EGR yes, the injector probably not.