Have a read through
this thread, should give you some pointers.
Basically:
Check the vac pipes are connected and not split
Check the VNT actuator arm is moving freely
Check it holds vacuum - if it doesnt the diaphragm inside could be split
Check boost pipes and clips
Check the N75 valve is working - output test in vagcom - this will move the actuator
If all of the above are OK it is more than likely sticking VNT, so log the boost levels and see if you are getting overboost that is throwing the limp mode.
Neil