The volume increase in the radio could be caused by the automatic volume
control feature of the radio. It uses the VSS (vehicle speed sensor) to
increase volume automatically when driving at higher speeds. Try turning off
that feature (on my radio the option is in the menu under AVC) and see if
the volume spikes when the speedometer goes crazy again. If the volume
doesn't spike, then something might be wrong with the VSS or some VSS
connections.

The speedometer display is driven by the ECU. This takes a value from the
VSS (Vehicle Speed Sensor) that is mounted on the transmission. The radio
has an option to get louder as you drive faster and this also uses the
value from the VSS.

It is very common on older cars (mine included!) for the VSS to fail such
that there is no speedo display.

Your problem would seem to be either an intermittent connection, probably at
the VSS connector, or interference from another vehicle system. The VSS is
fairly inaccessible, and is reached from under the vehicle. If your vehicle
is under warranty it should be an easy and quick fix. If you need to do it
yourself, or point someone else in the right direction, try Googling in
this group for "VSS" or "speedometer" and you will find out lots of info.

If your VSS should need replacing, it costs about 25UKP to buy and some
patience to replace. A main dealer would charge up to 100UKP to do it, an
independent a touch less. No special tools are needed.

For future reference please let the group know how you get on.

Good luck and enjoy your Focus!

Chris