My 2000 Chevy Impala has a rough, shaky idle when stopped at red light

What is your question?

I have a rough idle and very poor gas mileage on my 2000 Chevy impala.I just replaced the spark plugs, now code says it's a cylinder one misfire Bought car used in July had only 68,000 miles ran good then, a month ago it started rough idling occasionally but now it's all the time. I forgot to ask owners what fuel they were using, could that be part of the problem ? Car makes slight thumping noise when in reverse and rough idle and slight shaking at stoplights but does not stall out. What should I do next ? Thanks

What makes this problem better or worse?

Worse when engine is warm

How long has your 2000 Chevrolet Impala had this problem?

A few weeks