2002 sentra mileage 30,500

january of this year my car would studder while i was driving it but only randomly, then in april of this year i got into a car accident and my car was "repaired" when i drove it off the lot the car has done that same studdering ever since and it just gets worse and worse. my check engine light came on got it checked and it says misfire cylinder 1. the code was p0301. i took it back to another shop and they changed out my coil pack but it still does the studdering thing. it studders and sometimes lag when im pushing down on the gas. does anyone have any ideas what this could be if so please help me out.