Lets start out with some basic info first. Has the check engine light come on? And if the light comes on do you see it flashing when it starts to sputter? So your first step should be to have it scanned for fault codes. I would recommend calling Volvo to see if they are participating in the Volvo service for life program first? Just tell them the check engine light comes on and off and you would like the complimentary diagnostic done on your vehicle.
This is what you will get. You will get a free scan and base on what the codes are there maybe a quick test that needs to be done. If the fault can not be found during the quick test then they will advise you that they need more time and will advise you up front that you need to spend some money to go in deeper. If it is a common fault and they already know what is wrong then they will quote you parts and labor for the repair. It is up to you if you want to proceed or not and you can always decline. At minimum you will get the actual fault codes stored and ask for a copy of the print out of codes. Once you have that post them on here to see if we can figure what is going on. To be honest I would rather you get Volvo to scan it for codes then using a generic code reader since sometimes there are other codes stored on there that a $100 code reader wont pick up. So do that first and get back to me please. If the check engine light is flashing then let me know. That means you have a sever misfire in one or more of the cylinders. But sometimes I have seen them not post the light if it misfire say only while under load? Good luck and hope to hear from you soon.