mine did the same thing. turned out to be a problem in the cluster. could be a resistor going bad or a solder trace breaking, after youve driven it for 20 minutes it heats up and shorts it out. had to get a different cluster to fix mine.
best way to tell if its the cluster or vss, drive it and set cruise control. when the spedo acts up if the cruise is still working fine and doesnt become erratic then its the cluster.