Still have power to the warning lights and indicators? Sounds like power and speedo signals are still getting in there. Not sure about the '02s, but there are plenty of places for a loose signal/power to the display
Power and signal points
I'm not advocating you open the speedo up (it's a pain), but when it happens again -- and you're stopped--give the wires feeding the speedo housing a giggle to see if something changes.