usually it happens when i drive 40mph sometimes the needle in the speedometer drops all the way down to 0mphs for a second or two. then it gets up to 40.
also sometimes it drops down to 20mph and then goes back up.
last time i even had problem with my Sporting light it started to blink after 15-20 seconds i stopped car and turned off engine then after few seconds I strated engine again and everything was fine

 

most likely is the electrical part of the speed sensor, i have seen many of them go bad but make sure all wiring and connections are good

 

have a 90 accord, it did the same thing to me.

started off with intermittent problems...went away, came back after 2 months and after jumping for a bit, it went dead ( 0mph ).

replacing the speed-sensor resovled the problem.

i agree with deserthonda, check the wiring first, clean the contact and see if it makes any difference.

good luck.