[Mike]

This doesn't sound like a speed related volume change if it is fine when you stop but only changes on startup.
I have noticed that if I change the volume using the steering wheel controls then the change in volume is not retained on startup, but if I change via the central knob then the change is retained on startup.
This is a feature I like as I can set the default volume and know it will always stay at the same level as long as I use the steering wheel controls for temporary changes.