As it turns out several things can contribute. Vacuum loss affecting the pressure regulator being just one of them. A plugged 'sock' at the pump is another. But probably not from 45 PSI to 15 PSI.
I checked the fuel pressure on a mid '90s GM LT1 and it's about 15 PSI. It should be around 45 PSI I believe. What could cause that? Failing fuel pump? Obstructed fuel filter? I'm baffled.