Eeewww .... that's bad. Once I had a problem like this I believe. It turned out that there is a crankshaft position sensor whose wire is draped over the top of the transmission. Years of running the car had vibrated the insulation off of a portion of the wire leading to the sensor. The uninsulated portion of the wire used to touch the transmission case and short out, cutting the power to the motor. Then it would seemingly reset somewhere and start up again. I'm shooting in the dark here and this wire may be well secured on your car. But Volvo stories are many and this is one