Hi Jack, are you sure the cold air feed is fitted correctly? I ask this because if there is even a small gap allowing air in or out past the maf sensor then this could be causing byiur issue.
In failing that I'd advise byou get a diagnostic check carried out to see what fault codes flag up. As a correctly fitted cold air feed that is designed bfor your Audi shouldn't cause the car to go into limp mode.
Cheers
Steve