Something in the vehicle speed sensor circuit is grounding out. Check out your speed sensor on the back of the tranny, follow the line and make sure there are no nicks or frays. Get some electrical contact cleaner and spray out the plug and receptical, followed by drying with compressed air. I have noticed that they can cause problems when they get wet. They are high enough up in the bay that when it's raining they don't get too wet, but if you washed the bay, or lost a coolant line or had a leak that could be it.