Speedometer needle moves when car stopped

2002 Taurus SES with Vulcan engine and 39k miles. Wierd thing happened today at red light. I noticed my speedometer needle bouncing around between 0 and 5 mph while stopped at the red light. When light changed and I took off it was steady at proper speed and behaved normally after that at subseqent stops.

Is this some strange one time occurance or is something going bad. What would go bad to make the speedometer needle "jittery" when car is stopped and idling in drive?

Thanks.