2006 F150 Rough starting.

I have a 2006 F150 STX V6. Sometimes I have trouble getting it to start, I will turn the key and it will stay there trying to crank but just wont start up and when it does crank after a try or two, it will be a rough start and make the truck shake rough. And then other times it will crank up with no problem. I already changed the spark plugs and plug wires, What else could it be?