I had almost the exact same problem with my 04 S60R. I took it to the dealership since it was still under warranty. Turns out I wasn't the only one with the problem. There is a recall. I would suggest subscribing for free to the following website. http://www.mycarstats.com
It indicates the following in the recalls section:
Component: ENGINE AND ENGINE COOLING
Details: ON CERTAIN PASSENGER VEHICLES, THE FUEL PRESSURE SENSOR LOCATED ON THE LEFT END OF THE FUEL RAIL MAY, AT TIMES, TRANSMIT INCORRECT SIGNAL REGARDING THE FUEL PRESSURE TO THE ENGINE CONTROL MODULE. IF THE SIGNAL IS OUTSIDE OF THE PRE-PROGRAMMED ALLOWABLE LIMITS, A DIAGNOSTIC TROUBLE CODE MAY BE SET AND THE CHECK ENGINE LIGHT WILL COME ON. THE SOLDERED JOINTS ON THE CIRCUIT BOARD OF THE FUEL PRESSURE SENSOR MAY CRACK DUE TO TEMPERATURE CHANGES AND EXCESSIVE VIBRATIONS.
Consequence: MISFIRE MAY OCCUR DURING DRIVING THAT, IN TURN, WILL REDUCE THE ENGINE TORQUE AND IN THE WORST CASE SCENARIO, THE ENGINE MAY STALL WITHOUT WARNING. IN CERTAIN TRAFFIC SITUATIONS THIS COULD CAUSE AN UNSAFE SITUATION AND POSSIBLY A CRASH.
Corrective Action: DEALERS WILL REPLACE THE FUEL PRESSURE SENSOR FREE OF CHARGE. THE RECALL BEGAN ON JUNE 29, 2007. OWNERS MAY CONTACT VOLVO AT 1-800-458-1552.
Units Potentially Affected: 38700
Recall Initiated By: MFR
Date added to NHTSA datbase: 06/05/2007
I would definitely take it to the dealer they will fix for free. Don't take no for an answer, expecially as it is a safety concern.