2006 Jeep Laredo WON'T TURN OVER ALL THE TIME

I have a 2006 Jeep Grand Cherokee Laredo, 6 cy, 4 wheel drive. Some times when I go to start the Jeep it will act like it is going to turn over, all the bells and whistles come on and it acts like it is going to start and then it doesn't. I had the problem 5 wks ago the first time but when I took it in, they started it fine and had it for 3 days and it never did it to them. They figured it needed a new battery so they replaced the battery with a Jeep Factory battery. Then last Friday night it did it to me again. A friend was able to start it right up and then it started fine all weekend and hasn't done it again.