Probably something from the speedo itself in the movement binding the needle or the drum. Most likely culprit is the return spring from the service reset board lever. Other possibilities are the broken peg, also from the service reminder reset lever spring (that's a tough one to see as it's just a little piece of clear plastic), pieces of old, broken odometer drive gears, and, as of yesterday a first for me, a dead earwig.

If you replace the speedo, it must be from an ABS car to match, 92-93.

DS