Could be the pressure regulator or the fuel pump check valve. You should be getting a lot more than 8psi on the the key cycle and I wonder if the pump is going away. Unlikely injectors would leak that much, it would be flooded and not start at all. It should hold pressure for quite a long time. To isolate, you can pinch or block the return line to see if it's the regulator. If the pressure holds it's the pump check, if not suspect the pressure regulator.
At idle, failing pump might keep up but not be delivering properly at higher demand. I'd watch it under a road test too. If the vacuum to the regulator is working, the fuel pressure should be stable under varying vacuum loads and you should road test it to confirm. But, first test it with hand pump to see if fuel rail pressure changes with the hand pump vacuum.
When's the last time you changed the fuel pump filter? I've had one blocked so bad, I couldn't blow through it (and a new one blows like it's a straw). Don't under estimate the importance of a blocked filter, I did once and it cost me a pump.
I assume it's stock because I switched to an adjustable Aeromotive style and it bleeds down within a few seconds too. Bothered me when I first put it in but when I called them, that's normal. These days I use their fuel pump controller so I pressurize via the override before cranking and have set the speed controller ramp so it the pressure regulator isn't working much below 3,000 (when it goes to full speed anyway).
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