Fuel Injection problems of the 2004 Volvo S60

I have a Volvo S60 2004. Around January 2007, it started stalling without warning in the middle of the highway at 60 mph. It did this about 3 times in 2 weeks, so I took it to the Volvo dealer. They couldn't reproduce the problem, but supposedly upgraded the software and cleaned the throttle intake, and returned the car to me. The car was fine until March 6, when the same problem occurred. The car is currently back at the shop where they have told me they have found nothing wrong and are going to return the car to me. I would not like to get killed on the highway, and as I have seen that this has been reported before on other internet sites, its worth reporting here.