2006 Mustang GT problems starting after getting gas

I have an '06 GT it has around 60,000 miles on it, last week i filled it from almost empty and it wouldnt start for about a minute. it finally started and was fine after. it was starting fine after so i idnt think anything of it. sunday i put 10 bucks in and it just sputtered when it started, now today i put 30 ucks in and it didnt wanna start for about thirty seconds then was really lazy for a few seconds after that. i got it home and shut it off, took of the gas cap put it back on and it fired right up. what would be doing this. any suggetions?