Feels like engine is chugging

I recently had some problems with this car. I bought it used. One owner and only 21,000 miles. Recently when I drive it for about fifteen or twenty minutes it starts to feel like the car is almost dying without shutting off. Meaning it just feels like it's "chugging" for lack of a better term to explain it. I have had it looked at and no codes are coming up. Just cannot figure out what it is that is wrong.