2003 Hyundai Accent

my car hesitates intermittently when accelerating. when my mechanic test drove the car, three times, it did not cause a problem. what can it be? it actually feels like you are hitting the brakes when accelerating. it jerks like that. does not do it all the time. same days it seems worse others it does not do it all. Tried different gasolines, had a tune up recently.

I will try to make a donation if you answer my question