2003 Ford Escort 2003 ZX2 Hesitation Up Hills & Gas Mil

I recently just bought a 2003 ZX2 and did a complete tune up, PLUGS, WIRES, COIL, OIL CHANGE AND FILTER, AIR FILTER AND FUEL FILTER. It has been running a LOT better but my gas mileage is still horrible! The tune up helped another 10% in fuel econony, it is a 12 gallon tank and iM getting almost 300 miles to full tank. Its DOHC and I know it will use more gas if you get on it, but I have been driving it real nice still breaking her in! Up hills it has no power and it feels like you have to push the pedal to the floor just to down shift it, it feels like iM losing power just up hills though! Down the highway its fine until the tranny shifts into over drive and all of a sudden at cruising speed, at very low RPS below 2000! I will feel just the slightest bit of hesitation in the foot pedal and in my seat, whats HAPPENING? Whats are some other things I could check or replace cause the horrible GAS MILEAGE, HESITATION AT LOW RPM'S and LOSS OF POWER when GOING UP HILLS?