2001 Mercery Grand Marquis LS. Has 122,000+ miles on it. The speedometer will intermitently register a Zero in place of actual speed. When this happens: Odometer does not advance, Miles remaining in gas does not reduce, Miles per gallon reads 99.9, Cruise control still functions and now the Trunk Ajar light and audio goes off every now and then. Check engine light does not illuminate. Used to coincide with bumps in road, but not now. I can smack either the dash above the digital dash board or the steering column below it when this happens and the MPH will jump but return back to Zero. Ford dealership said it was either the speed sensor or the sensor cluster but could not duplicate for verification. Any help? I know I can get a refurbished digital dash from advanced auto parts but want to make sure this is the likely problem instead of a sympton of a loose wire/connection. Also, Parts store said they only had a refurbished dash rated to 120 MPH. I don't know which I have but when I bought it used, it had a factory installed high performance exhause on it.