I have this problem as well on a 100,000 V40 I have just purchased. I am going to try a change of HT leads to if this sorts it, but any views are very welcome.
Some improvement, the the problems is still there. Took it to the Volvo dealer and they said the MAF sensor was reportting an error code, so I am going to replace this next. Let me know if you have any luck fixing the fault.