I just purchased a 99 catera from a private owner has 84000 miles on it. When i first got it home it had a engine misfire 300 302 303 304 ran rough sometimes stalling or pressing on the gas it would not go just backfires real bad also fluctuates in rpms. i changed coil packs and spark plugs nothing changed but misfire was now 300 301 303 305 then changed fuel filter still no change misfire is 300 302 303. Also is making loud ticking noise under passenger side valve cover. Has anyone experienced these problems or know any solutions?