1998 safari

My 1998 safari van with a 4.3 has a no power problem for the first 30 seconds every time you start the engine. after 30 seconds it clears up and runs ok.
Any ideas?

Check fuel pressure and leakdown rate. Ignition in RUN position, engine off, fuel pump running: Pressure must be 60psi to 66psi. Pressure must remain above 55psi for 3 to 5 minutes after the fuel pump shuts off. Post your results.