Inductance and RFI interference from the plugs , wires and other power sources can cause all kinds of grief with those dakota digital dashes..
Get the wires that come from the speedo pulse generator away from the plug wires and any other wires that carry current. (the firewall won't keep the inductance away) and try not to run the wires from the alternator in the same bundle as the ribbon cable for the dash or sending units.. I'd start with the speedo wires though.
That's what I had to do the last time I had a prob with the tach and speedo acting strange in a 41 plymouth with a dakota digital dash.
Now what language is this?...
When I have the KOEO ti rears fun...