My 2004 S60 does the same thing. Recently it started getting worse. The engine would shut off while initiating a turn disabling the brakes and power steering... really scary stuff. I took Rmikers advice and cleaned the MAFS and test drove it. It was a good idea but it didn't help. I noticed that only shuts off when initiating a turn so I decided to experiment. I discovered that if you are braking into a turn while your indicator light is on, while your seat warmers are on (and possibly even the rear defrost - normal conditions during winter) the car will shut off. The condition can't be caught by a diagnostics computer. If you pay the $160 for the dealer to run a check, all their reports will come back fine. It's an anomaly. The interesting (yet slightly disturbing) thing is that S60s all across the nation from Florida to Washington State to California are all having the same issue pop up about 80 - 100,000 miles into the cars life span. It is Volvos responsibility to initiate a recall objective for these models. Volvo built their brand on safety. If Volvo is aware of this condition - and one person dies because of it - their remarkable branding would surely be in jeopardy.
Its a problem with the mass air flow sensor eye. You can by cleaner at your local auto shop or just simply replace the part.
If you want to do a short bypass in the mean time till you get your materials you can unplugg the sensor.
You may see a difference in your gas efficiency but, like I said it would only be for time being. If it still does not work. I would
Your fuel pump sensor.