Your problem could be the VSS or the speedometer.

When the speedometer isn't working, try to use the cruise control. If it works, the VSS is ok and your speedometer is bad. Mine would get stuck at ~19mph and the odometer wouldn't work. The cruise would work fine. It is a common problem. You can try remelting the solder joints on the speedometer board. You would have to remove the gauge cluster and remove the speedometer. I read this fix on another forum and it worked for someone on here.

A bad VSS will also turn on the check engine light. The D4 light would start flashing as well.