I've just tried swapping back the MK III nav and now have interference with that but I might have jumped the gun earlier.

It seems that the interference happens when the car is moved/moving and the screen flickers and the colours change as well, but it's not dependant on the engine running.

The only thing I did when fitting the new nav was to swap over the 2 connectors and co-ax cable so wondering if maybe the connectors into the back of the unit might have worn a bit giving some movement so that any vibration from the car is causing a slightly bad connection and causing the flicker - does this sound possible and if so any suggestion how to fix it?