I would agree with Tabetha , sounds more like a vehicle speed sensor issue , but with the fiesta this is fed to the Engine Control Module by the ABS module using CANBUS High and CANBUS Low , I would say you may well have a issue with the network , as tabetha says loose connection or a faulty wire not giving a constant connection , don't forget canbus work in digital format so they have to have VERY good connections to be able to transmit the data as required .

The only other issue is that something else on the CANBUS network is causing the signal to become distorted in some way , if you take your car to the ford dealership again ask them to do an active test , check that the output from each wheel speed sensor is the same , as the output to the DTC socket from the ABS module is shared with the speedo and ECM so should give you a good idea about what is going on