This same thing happened to my '92 940. My speedo was inop and the needle would either stay at zero or jump to 45mph and stay there till this weekend when it suddenly seemed to work normally. The odo and trip meter work as well.
Even the cruise control works. Things I did to fix it w/o effect:

replaced the instrument cluster from ebay ($75), replaced the differential speed sensor ($95 from the local volvo stealership). I also traced the wires from the differential forward, no breaks. I am bewildered. The temps here in NC have been unusually cold this week so that is all that I can attribute the reason the speedo/cruise fix to.

If you figure this out please let me know. If you need a good used (turbo) cluster or a used (working) diff speed sensor let me know and I can make you a good deal.
BTW, my ABS light sometimes comes on too. It is unrelated to the speedo problem. Check you abs sensors at the front brakes. They may need to be cleaned off. If that fails, a small piece of electrical tape to the instrument cluster works well.

herb
NC