The line from the fuel pressure regulator to the air resonator is actually a vent line and never has a true vacuum because it is in front of the throttle body and will never see much vacuum unless the air filter is totally plugged. My (Haynes) manual says 50-60 PSIG key on/engine off so your pressure is ok too. I doubt you have any fuel supply problem.
You could have a vacuum problem related to leaks on top of the fuel tank either at the roll-over valve or fuel pump module. More likely would be something like the vapor purge or vapor vent solenoid failed open. On my '04 the vapor purge valve failed open and caused some minor start & idle problems. As I remember a new Delco purge valve was under $20.00 @ rockauto.com. It is located on the side of the block under the intake manifold & you will need to remove the left front tire to access it. Personally I wouldn't go to the bother of testing it, just replace it.
Good Luck.

 

I had this problem last year. I had to change my fuel pump and put a new gasket on it. If its an original fuel pump they used a metal construction and they corroded and have tiny leaks sometimes. Newer pumps are plastic. If you put a new pump in do NOT reuse the old rubber gasket, get a new one it should come with a new pump.

 

I have a leaky seal at the fuel pump on the 355...
The new AC Delco pump is plastic and does come with a new gasket / seal.
Good luck with the fix!!!