My wife's vehicle chugged a few times on the freeway recently, slowing down a bit but ultimately kept going normally after the (several seconds worth of) chugging went away.

A trouble code reader for the vehicle (Digital Can OBD2 by Innova) read the following:

Fault Code P0193 Fuel Rail Pressure Sensor High Input.

I have evidently entered the modern era now that I have used a code reader.

I'll update this thread as I go to share troubleshooting process for this problem, hopefully it can help others. And of course if anybody has wisdom to share, it's appreciated.

I ran a bottle of Red Line complete fuel system cleaner through the vehicle, but that didn't resolve the misfiring/stalling problem.

Also got in a Haynes manual for the car, but I didn't find anything in there to help diagnose. It's good to have anyway, though.

Internet research leads me to next check the fuel pressure sensor.

There are some known quality control problems associated with pressure sensors which cause misfiring and stalling behaviors.

So I'll replace that sensor and see if the problem resolves.

Fuel pressure sensor is a known problem to a couple of years. I recall reading a TSB on it a few years ago.
There are 3 recalls on the 2005 V50, you need to check with a dealer to find out if your car is one of them:

http://www.motortrend.com/cars/2005/volvo/v50/recalls/