I have recently had this same issue where my 2004 S60 R would hesitate, it backfires, stalls, and studders. I had my fuel pump changed a week ago because I smelled gas and that was covered under a recall. The problem continued and my car started stalling out under load (hard acceleration) and I ended up taking my car back a few days later and the dealer replaced the FPS which was not included in the recall (due to my VIN#). My car is still hesitating, stalling and backfiring. Anyone have any other ideas?