Problem with speedo and radio volume jumping wildly, together with engine codes 420 & 503

Hi it would be very much appreciated if anyone coudl help point me in the right direction as we are currently car-less and I really need to sort this. My Ford Fiesta V 2003 MK V 1.25 16V has one or more problems: The radio will periodically go very loud, and the speedo either drop to zero or start showing a much higher speed than I am travelling. The car is also generating a P0503 (Vehicle Speed Sensor Intermittent/Erratic/High), which I assume is related to the problem with the speedo, and a P0420 code (Catalyst System Efficiency Below Threshold Bank 1) which I think could be a related electrical problem as we have had a new catalytic converter within the last couple of years. I had been thinking it might be the voltage regulator, but am now wondering if it could be a wiring problem. If anyone has any similar experiences and can suggest where I might look first, that would be a great help,