sites.google.com/site/1810martin/home/b5-vnt-turbo...s

This is an excellent website that will help guide you through fault finding.

I very much doubt cleaning the turbo would make any differance, so would leave that for the time being. What happens is that gunk builds up around the variable vanes in the turbo, and the vanes stick open > hence overboost/turbo shut down. Your code relates to underboost, so far more likely something is leaking air.

Chances are you have a leak in one of the vac pipes. Failing that a leak elsewhere in one of the air pipes that feed the turbo, or you may have a split in the turbo actuator diaphragm (unlikely if you're still on the original turbo, as it's the cheap aftermarket actuators which are fitted during a turbo-refurb that split more prematurely compared to the original e.g. Garret actuator diaphragms).

Check/replace all the vac pipes first. You can buy silicone tubing off ebay to replace the vac pipes (and far cheaper than what VAG sell) - you'll need 3mm and 5mm internal dia. tubing.

For future ref - As well as the common failure of the flexible vacuum pipes (as per this fault) it is also worth checking the ridgid plastic pipes (pleanty of these crossing the bulkhead) as if these rub/vibrate against anything they eventually wear through. I had this happen on my Skoda Superb producing similar symptoms. Yet again the ridgid pipe was cut short and the flexible pipe re-attached. The other thing that can fail are the vacuum solenoids (1 for EGR, 1 for Turbo vanes and 1 for aniti shudder) .