Evening, I have a 2013 a3 8v 1,8 stronic (17k miles) which is stock other than a pedal box and a cat back exhaust (pipe was cut betweenthe middle silencer and cat. This exhaust mod was done just before christmas but since the past 2 weeks the car has been quite 'rough' and 'shaky' during cold starts. Could the weather or the exhaust be a reason for this? or something else? any help appreciated..