Sounds like its going into limp mode, and needs the ecu read. It might bring up an error code 17965 Positive deviation - relating to boost. This is common, and used to happen on my passat, and is caused when the soot builds up in the turbo and causes the parts inside the turbo (VNT) that direct the flow in, or around the turbine to stick, causing too much boost created, and the ECU to panic.

You can buy basic copies of VAGCOM cheaply off ebay that will enable you to read the faults but no more...

Tom