IMO, based on the consistency of your symptoms the odds are high it’s the fuel pressure regulator. See the extended FAQ file on that topic
Volvo 700/900 FAQ: Fuel Pressure
In your case there may or may not be running issues as noted in the FAQ, but likely it will be running rich, your gas mileage will be poor and you may well find the plugs are uniformly quite sooty. As per the FAQ there are pressure tests you can do to check the FPR, but in your case I'd go straight for replacement unless you've got the necessary fuel rail pressure gauge to do the tests properly.
Less likely and only if you tend to run on the cheapest gas with minimal additives and/or the pickup screen in the tank is dislodged and/or the car has gone through extended periods of disuse would I suspect a fouled check ball valve in the fuel pump. You could try running a number of tank fulls with a high quality injector/fuel system cleaner additive to see if there are notable signs of improvement. If the pressure can leak down enough in half a day to delay cold startup to the extent you describe then I rather suspect such cleaners wouldn’t be up to the job. Anecdotally, people don’t seem to have much success restoring check valve functionality just with cleaners.
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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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