Your anti slosh could be acting up or dying. It's that little pc board gismo in the top left of the cluster looking at it from the back, Speedo is totally separate. The cable could have slipped out of the back of the speedo. Not real likely but maybe. It could be the drive gear/driven gear in the tranny lost all it's teeth...plastic ya know. Or, the speedos do just give up the ghost occasionally. I'd check the gears on the tranny end of the cable first.