I have a 2006 CTS 2.8L that won't start. The battery is new, the engine does turn over and after a few seconds wants to start but won't. It's almost like its not getting enough gas. This happened 2 days in a row then I had it towed to the dealership where it sat for another day. When the mechanic went to start the car today, it started right up and there were no diagostic codes when checked. Any thoughts on what might cause this?