2006 Nissan Altima 2.5S 4 Cyl

Hello, I recently bought a used 2006 Altima and as soon as I put the first tank of gas in I got a check engine soon light. Autozone read the code for me and said I had cylinder 3 misfire. I changed the plugs and at the end of that tank of gas the light went out. Next tank of gas same light comes on after filling up at different gas station. Autozone reads the code as multiple misfires (not a specific cylinder). I put in a can of sea foam and the light went out again towards then of the tank. Next fill up the light comes back on and again multiple misfires (not a specific cylinder). Gas mileage has been improving after each new tank of gas. Previous owner said all the emission switches were recently replaced. I am at a loss as to what to do next. Any ideas or suggestions are welcome!!! 139,000 miles on the car and I have only put non-ethanol gas in it since I bought the car.

Thanks! Joe P.