Occasional stuttering

I have owned this car for a little over a month. It ran fine first few weeks. We changed spark plugs they were not getting spark hardly, fuel filter which was really black, air filter, oil change an new filter which was overdue, front struts an brake pads. After we changed the spark plugs and I had gotten gas at a small Exxon gas station few weeks ago, after that it has been acting weird. It was just here an there at first but now it is a lot more frequent. It is hard to describe but It is like a stutter at idle or sometimes in park it shakes the whole car. We had checked the spark plugs to make sure they were right and they were fine. We have put fuel treatment in it with a different grade of fuel and it did a little better at first but then it has been doing it really bad past few days even know while driving occasionally it will jolt whole car it feels like an stutter at almost every stop. Today I heard a knocking noise come from tank area when it was sitting running for little bit stuttering off an on. I am really sure it is but Is this the fuel pump going out for sure or something else? I am not sure how to test this to be certain before it is changed.