2001 chevy Silverado has random spells of not cranking
 

I have a 2001 Chevy Silverado 5.3 that has random days it doesn’t want to crank properly. As of two months ago it wouldn’t start on first try but fire right up on second try. It would only do that once every two weeks or so but then it quit and worked fine since. As of this morning it takes 8-10 seconds for it to turn over and even then it stumbles but eventually runs like it should. Fuel pump is new, pressure regulator has been changed, and all injectors are new. I would really appreciate the help in finding my problem.