If your speedometer stops working and then starts again, you most likely have an electronic problem such as a bad
transistor in the speedometer receiving unit. This can be further confirmed if the odometer also has the same problem.
The Volvo 700 series speed runs off an impulse from a sensor in the differential, the clearence between the sensor can become covered with sludge and metal filings from the diff. The gap is critical.
Another possibility could be a short or failing component in the dashboard instrument cluster.