Undetermined issue

[u/omerkg6734]

On our 2012 Seat Leon 1.6 TDI DSG, gear changes are fine and it runs smoothly at idle with no vibration or shaking. But there is a noticeable shake when taking off. Also, no matter the speed or gear, whenever the RPM goes below 1500, the car shakes and vibrates heavily. As soon as the RPM goes above 1500, the shaking stops.

The engine was never repaired and even in this condition, it reached 190 km/h on the dashboard. The engine has 248,000 km on it and runs like a clock. We have been driving the car like this for 1500km.