See Below Link, Regarding Engine Issues;
http://www.golfgtiforum.co.uk/index.php?topic=262793.0How to test DSG on Testdrive;
Take the car for a good long test drive. The DSG will cause an issue when it gets to operating temperature.
When setting off from stand still the car should not jerk. Also a way to tell is drop it in reverse and it should not kangaroo in reverse too.
Also say your doing 30MPH (In D), The Gearbox sill probably be in 5th Gear cruising.
Slowly apply the brakes and let the car come to a stop gently. The DSG should drop gears very smoothly.
If there is a jerk say from 3rd going into 2nd then the DSG Mechatronic Unit is on its way out.
However these issues don't normally relate to the DSG Mechatronic Unit on its way out. Sometimes all you need is a DSG calibration (Reseting the DSG).
Also it's always good to drive in D Mode, S Mode and also use the paddles.
Also I would like to add the Gearbox itself is strong and very well built. The Mechatronic Unit is what are common to go.
I've also heard the clutch pack can warp causing the jerkiness.
Hope it help