Forget about the usual tests for speedo accuracy. That needle-not-coming-to-rest-when-stopped part is significant. Your electronic speedo head is going goofy. The common issue with all things electronic in a Volvo (not matter whether ALPS, Bosch, Siemens, or other) is failing solder connections on controller PCBs. In other words, most likely inside the speedo head itself. A bad ground circuit to the dash MAY be able to cause this, but other gauges would be affected too.

If you know how to solder, or know someone that does, the speedo head can be repaired after removing it from the dash cluster. Otherwise, a used speedo from a boneyard or eBay would be your best alternative.