My Smart car diesel is going very slow like it has no power / intermittent power loss. [new 451 cdi 2007 onwards]

The issue is usually a transfer of vibrations to the pressure sensor of the EGR (Exhaust Gas Recirculation) unit. The remedy is to fit a bracket to hold the components apart a minimum distance of 10mm thus reducing vibration transference and component failure / malfunction (main dealer if in warranty) and there should be parts available to rectify this from your dealer.