I have a 1978 Datsun 280z.

So the issue is sometimes my car instantly starts up with the turn of the key and then the other 90% of the time it takes about 5 seconds of holding down the key to get it to fire up and then it idles really low until I give it some gas and let it warm up.

My boss said it could have something to do with the fuel pressure not being strong enough (As in if there is an air leak maybe it allows the vacuum for the fuel to go back down and it needs to be primed again.)

What I don't understand is why sometimes it's instant!? And it has never been at the same time; as in I don't believe it has to do with the weather or how long it sits. Sometimes it will be started right after I turn it off and it still takes the 5 seconds other times it's instant. Then same with if I don't start in a couple days and then start it sometimes it takes the 5 seconds and other times instant.

And I don't believe it involves the starter or the battery at all. The starter was just replaced and it does the same exact thing and the battery is always charged but even in its lower power state will do the same things.

The fuel pressure regulator should keep fuel in the fuel rail. If the FPR is leaking that pressure out it'll take a minute to prime the lines back up, also parking on a hill will make this worse.

Where is the fuel pressure reg on the car? And where would you think it would leak on it?

The FPR is attached to the fuel rail and the fuel return hose attaches to it. If it's leaking internally it just lets the pressure pass through the fuel line, no external leak to detect. Then the pump has to re-pressurize the lines when you try to restart.