Engine periodically misfires

Several months ago while driving on freeway, engine started hesitating, mission. The Engine check light flashed for a moment. I cleaned air intake and air flow sensor. Ran code that said no.2 cylinder was misfiring. I changed plugs and ignition coil on no.2. No problem for several weeks, then while driving it started all over again, but extreme misfiring. Drove car to Pepboy's to check, they couldn't get to it, started call and everything was normal. What is happening? Money is very tight.