Why does a 1999 Buick Regal GS only have fuel pressure at key on and then looses it imediatley after starts and runs for 3 to 5 sec then bounces right back up after engine stalls?

The GS has a two-speed fuel pump, which is controlled by the pcm through 2 relays. During cranking, full power is supplied to the fuel pump through one of the relays. After start, when the ignition switch is relaxed into the "run" position, the pcm then delivers fuel pump power through the other relay. That relay sends the power through a large resistor that looks like a ceramic horseshoe that is mounted on the frame under the battery at the right front of the vehicle. You will find the connector behind the right headlight assembly. It has 2 wires, and is fairly easy to find. You can ohm it out from there. Most likely that is your problem. The GM replacement comes with a relocation kit, so it can be mounted above the engine on the firewall in a friendlier environment. If you choose to relocate it, you really don't need to remove the old one- it's kind of in a tight place, and not real easy to find.