Don’t know but you may have to check the voltage level being supplied to the unit itself to determine whether it is in the unit or in the power supply path to the unit.
No doubt this will be difficult if it only occurs whilst driving. It sounds like a loose (high resistance?) connection that is being affected more by the movement of the vehicle
Here’s the Range Rover Sport L320 2005-2008 Repair Manual +Wiring Diagrams
Scroll down and select the Electrical Wiring Diagrams link suitable to your particular model. Once the file has opened you will have to search through it and find the power supply wiring path for the radio/nav unit. At least once you know which harness connectors the power goes through for the unit you may be able to inspect them first and then somehow contrive to insert the leads of a Voltmeter into the appropriate connector and check what the voltage is at that point when driving. If it doesn’t alter then move on to the next connector etc, until you’re at the radio/nav unit
Get back here if you need more help. Supplying which electrical wiring diagram is the correct one to use will also help ;-)