2003 Cadillac CTS sometimes won, t start

I HAVE A CTS AND ONCE AND AWHILE IT WILL NOT START THE STARTER CRANKS BUT WILL NOT START I WILL PLAY WITH IT FOR ABOUT A HALF HOUR THEN ALL OF A SUDDEN IT STARTS I TOOK IT TO A DEALER AND THEY SAID THEY COUD' NT FIND ANY THING WRONG. THE NEXT DAY IT WOULD DO IT AGAIN. A FRIEND OF MINE SAID HE THOUGH IT WAS THE FULE PRESURE REGULATER IF YOU AGREE WHERE IS IT LOCATED