Could be a lot of different reasons. I had a bad gas problem years ago, but in my case my engine would hardly run at all after fueling up (whole lot of spitting and sputtering and shaking going on) . I used a can of dry gas and changed the fuel filter (Fram frame mount type) and the problem went away after about 15 minutes of testing.

But I won't go so far as to say that bad gas is your only problem.