This problem is why most of the old time Volvo owners/drivers carry spare parts with them when they go anywhere.
If I were you , I would buy a new fuel pump relay, good used ignition module, good used distrbutor (for Hall Sensor, cap, rotor button, etc).) and a used, known to be good, Jetronic Fuel injection module. Plus I would keep some spare fuses and a 12 volt test light in the car.
You can find all of these parts for very reasonable prices if you have the time to shop around.
Having plug and play parts are a big help when you're broken down on the side of the road.
I'm not saying that these cars break down more than other cars....actually they're much more dependable than the average automobile, and are a great source of low cost, comfortable, safe transportation (if you do your own work)....I'm just saying that most of the time the problem that will strand you is usually very simple but sometimes can be a real bear to find.
Especially the type of problem that you're having...most of the time it runs great until you're driving along and it quits for no apparent reason.
All of us old time Volvo guys love these cars and know most of the little quirks that can cause you to call for a ride. If you will take the time to learn about them,(this web site is a great place to learn) your life will be much easier if you drive a Brick.
Just my opinion
steve