that cutting out and then restarting after 5 minutes, That is a no brainer usually. your car has the correct mileage and age for that to simply be the fuel pump relay!
No need for a shop! Just a torx driver and a new Volvo relay!
for fun you can pull the relay out! Gently pry the little plastic tabs away to lift off the relay cover! They get your lighted magnifier out and look for the very small not easy to see( at first )cracks that will show up in the solder joints at the circut board! the first to go are the most important! These are the joints were the coil wires attach to the board! Do not bother to re solder them! Yes it works for a short time! Just install a brand new $30 relay and go drive it!
The relay is in th ehot engine compartment! The car warms up and so does th elittle board inside the relay! When th eboard expands slightly th ecoil loses its source and drops out! Sometimes they might restart right away! Other times it takes minutes! Many shops charge millions trying to find this one as they can never recieve the car in the failed state! As you said it clears up and there are never any codes stored!
The 93 can be made trouble free with just a handfull of parts and more important a solid inspection by a volvo Nut!
Keep the 93 at least until you put some volvo miles on it! Those miles just barely have her broken in!
Also if you want to feel like a new car! I installed the 94 tail lights and washer nozzles on my last 93 then drove the tires off it!