You symptom is down to each and every letter identical to the problem I had with my D5 from 2012. It had nothing to do with the gearbox at all. Instead the fault was with the TMAP sensor. That's a sensor that's mounted on the outlet from the intercooler, where it measures the temperature and pressure of the air fed into the manifold.
My particular TMAP sensor claimed there was a charge pressure even before the engine was started. When the sensor reported a higher charge pressure than really present, it doesn't want to increase the power output from the engiine, since it's already supercharging, or so it thinks. Not until you hit the floor, or nearly so, is the desired pressure higher than the falsely reported one, and only then does the injection increase.
The car's diagnostic system doesn't detect this, so you get no error codes. The good thing is that it's much cheaper than a troublesome gearbox. My car recovered completely after changing the sensor.
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