Sounds like the classic signs of the vane ring stuck on the exhaust turbine of the turbo. You can cure it by removing the turbo and stripping it to clean it, or you could try Innotec Turbo Clean. A google search should enable you to find it. I've used it and it does work.

If the engine is going into limp mode from an over boost condition I'd expect it to log a P0234 error code. What scanner were you using ? If it was a generic EOBD cheapie then they sometimes won't read the errors. You'd be better off getting VAG COM plugged in as it's far more reliable in these cases.

As railroad said, it's possibly the VNT mechanism either sticking or seized but another possibility is the vacuum actuator sticking or diaphragm split ? Other possibilities are boost pressure sensor fault, split/leaking turbo hose or intercooler, even maybe the coolant temperature sensor ?

Get the error memory read on good diagnostic equipment and also read the actual values of the boost pressure, coolant temp etc as just scanning for codes is only half the story. You need to see what the values are when the fault occurs to give a clearer picture.

Do this first, then you'll have a better idea as where to start looking for the solution.

Good luck, and keep us posted.

Don't ignore physical checks - vacuum hoses, boost hoses.....check that the vane controller on the turbo can move through it's full range. If the vanes are coked up it will need a push with a screwdriver but don't force it.