This is also a symptom of the throttle body feedback potentiometer problem common to 2001 turbo engines. Check the label on the throttle body , visible below the intake manifold. If it's white, the throttle body has not ben replaced, if yellow it's a replacement unit. Volvo has an extended warranty for this part, 10 years/200,000 miles and a software upgrade that may help it. The software upgrade has to do with the idle speed feedback loop. An intake leak could also cause it, this includes all ducting etc downstream from the mass air flow sensor at the air cleaner. The NTSB has gotten involved and that's where the extra warranty came from. The car can also surge forward due to idle instability-my daughter's V-70(2001) does this.