1995 5.0l Intermittent Fuel Pressure

There are a few things that could cause the pressure to leak down... the regulator as you mention, a worn fuel pump bleeding down, and ( I 'think' the most likely thing is) possibly a sticking fuel injector, since you mention it's intermittent, and due to the age of our cars.

Symptoms of this would be the fast bleed down of fuel pressure, a lean condition in some cylinders (perhaps the whole bank), and a rich condition in the cylinder with the stuck-open injector. If you're getting a lean CEL (plus the pinging), but at the same time smell fuel or have white smoke coming out of the exhaust, it's very possible this is what's happening. If you're not, then the fuel pump or regulator are more suspect.

You could try running a cylinder balance test, checking your spark plug condition for rich / lean conditions, and replacing the fuel filter as quick & cheap diagnostics. If the O2 sensors are original, replacing those may be a good idea at 100k miles. Many will tell you to clean your MAF as ordinary maintenance too, it's probably not a bad idea to check the EGR valve condition too. Beyond that, I'd suspect the fuel pump since the regulator appears functional (if there's no fuel in the vacuum line, it's even more likely that it's working properly), and the pinging & lean condition seems to happen when fuel demand is high (up a hill).