1999 Pontiac Grand Prix Car starts then stalls

Hi. My 1999 Grand prix gtp with the supercharger will start. Then after 30 seconds to approx 3mins. The car dies out. It acts as if it running out of gas. It was running fine, then about 30 mins after filling my tank the probem started. I have replaced the fuel filter, pump and pressure regulator, but the problem still exsists. It holds 40psi of pressure, then it starts to go down slowly after idling. It goes down faster under load. It has no problem starting up, just staying going. It has also been checked wiith a portable scanner with no error codes showing. Any ideas or help would be greatly appreciated.