Ideas

I have a 2001 Cavalier Z24 and it has 139, 000 km. I have a starting problem sometimes, but it doesn't happen in any specific conditions/temperatures. It will work fine for about a month...great actually, with no problems and then it will decide to not start for 3-4 days. I have replaced spark plugs with nice iridium ones and checked boots and coils, which were both good condition. Would an old fuel filter cause a no start situation? It needs a new air filter too, but I'm gonna hold off because I want to buy an air intake for x-mas. Had a diagnostic service done, mechanic said he recommended fuel filter/spark plugs...but could gind nothing else specific. Any suggestions as to why this might happen would be great. Thanks, Matt.