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Both the check valve on the fuel pump and the fuel pressure regulator hold fuel pressure as well as the injectors.
If the injectors were leaking you'd probably notice a hard or rough start, so in your case that's very unlikely as you start right back up after priming pressure.
If the car has been in storage then stale gas varnish could be an aggravating factor. A few tanks of top tier gas and using an added injector cleaner, coupled with lots of ongoing highway driving may help slowly dissolve it and any other deposits.
The most common cause is a failing fuel pressure regulator. After allowing for fuel pressure to leak down and before starting, pull the line off the FPR and sniff into it for fuel vapours or even fluid, meaning a leaking diaphragm. The FPR could also be leaking back down the fuel return line. You could disconnect the return line from the fuel rail and use a small vial to see if it catches fuel leaking out overnight. If the leak is subtle it may soon evaporate. If the FPR hasn't been replaced in living memory then maybe just opt for replacement anyway, it's easy to replace. If it turns out it was okay then you now have a spare.
Next likely is the check valve on the main pump. It's a minor pain to get at and remove, so depending on my mood I'd probably start with a few tankfuls of super premium using an injector cleaner additive to see if that starts to improve things and carry on from there. If you don't see an improvement after a couple of tanks then it's more serious. After that I'd finally break down and remove the check valve for manual cleaning or replacement. The checkball inside may be worn and/or sticky and/or the spring failing. So far I've not needed to do that as I don't recall ever having a failed check valve, although others occasionally do. They're not expensive, just more difficult to access and remove. A flare wrench is recommended with a slightly offset counter hold wrench squeezed hard together in your hands, preceded by penetrating oil, and quite often what it takes to crack open brass fuel line fittings.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now
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