I had the same problem ith my 2001 v70 wagon. The dealer(after numerous complaints) put a computer box in my car that when the symptons ocurred I had to push a red button which was then recorded in the computer . Although it was a pain and I had this contraption in my car for a month, The problem was diagnosed as a bad transmision and Volvo put in a brand new one.
The symptoms were when rounding a corner the car would go into neutral the tach would shoot up to 4000 rpm, the car would go nowhere and then the transmission would kick in. It definitely was asafety issue so I would continue to complain and see if they can do the same for you. My problem was more sporadic but after two weeks with thw computer in my car the problem happened and the computer recorded the problem. BAD GEARS IN THE TRANNY. GOOD LUCK