Sure you've done these anyway, but worth a shot if you haven't:
Check all cables to/from battery are firmly connected, clean connections etc.
Check overall condition of battery - if you've got an indicator on it check it's green.. Battery may be on it's way out?
If you've got one, get a multimeter across battery terminals when engine is running to make sure Alternator is charging the battery properly.
It might not be linked to speed, but rather load on the battery. Maybe try turning stereo and other ancillaries on when sat idling (aircon as well) and see if you can replicate the
problem.
Failing that - get someone to do a VAG-COM scan... Might find something.
