1998 Chevy Astro FUEL DELIVERY

Hello,
Here is my problem: At regular fuel pressure of 58 psi the van starts up easily. After running for a short period of time the fuel pressure drops under 50 psi and stalls. Right before the van stalls, as the pressure drops, the fuel pump makes a lower pitch sound. With engine off fuel pressure doesn't drop it stays at either 58 or little above 40. When it stays at 40 psi it is imposibble to start the engine. After few atempts to start the van the sound of the fuel pump goes back to hi pitch and pressure goes back to 58 and the engine starts right up. I replaced the fuel filter and recently I also changed the fuel pump. Thanks for any suggestion. John