My 1990 iroc has a speedometer issue. It won't work until I drive it for about 20 minutes, then its a bit shaky then fine after that. This happens every time I drive it. Suggestions on what is happening and what to do to fix it?

 

Could be your speed sensor. Could be some corrosion in the contacts somewhere. Could be the stepper motor. Could be solder joints on the circuit board in the cluster going bad. Could be the circuit board components.

If you know someone with a shop scanner that can show data and not just read codes you could test this. A scanner would show if there is a reading from your speed sensor. If there is, and the speedometer doesn't work, it's probably somewhere in the cluster.

You could also pull the cluster and see if there is any obvious corrosion.