I have a 2004 mustang gt and the service engine soon light came on and there was spark plug #5 misfiring, is this a big deal, what can be done to fix this? thanks

 

Yes, check the spark plug........

 

could also be a vacuum leak

 

I am having the same problem i think, except mine is a 99. it only happens at high rpms, and if I am going uphill. Day to day putting around its fine. IF it is a vacuum leak what are my first steps to track down the leak.?? Its times like these i wish i would have paid more attention to my dad.