[V8] Ghost in the Speedometer?

[Speakoutprod at aol.com]

Yesterday while driving to work, I looked at the speedometer and, for about 
three seconds, it instantly dropped from 60 MPH to 0 MPG, then, after about 
three seconds, instantly went back to 60 MPH.   

(OK, maybe it was 70 MPH).

This is the only time I have observed this happening in the life of the car.  
Any thoughts what may have caused this?

Thanks,

Brad Hartman
1990 V8
168,000 miles