Car has trouble starting every once in a while

For like the past 6 months my car has had trouble starting every so often.  
At one point I checked the battery and it checked out OK.  Yet every so
often it will have a long start.  Right now it is about 1-2 times a week a start is
long.  Sounds like the starter turns it over just fine, but it acts like it isn't
combusting right away or like only 1 cylinder fires and the others struggle to
combust?  It is so strange and for the majority of the time it fires up great!  
Ever run into something like this?  It is 3.6L V6.