2000 ford zx2 escort

it idles gd but from zero to 30 m.p.h. as i go thru the first 3 gears it bucks +hesitates once i get to highway speed it runs great i have no engine codes on i just replaced throttle position senser with no luck any ideas ? thanks

Maybe fuel filter or pump.
Have fuel pressure checked and go from there.

How old are plugs, wires, air, fuel filter? Check fuel pressure as hd72mm said - best to hook up a gauge with a long hose that you can tape outside the windshield & have an asst watch it as you accelerate.