This is a similar problem to the T5 not boosting at all, and not
unusual.
For some strange reason, the boost pressure sensor is mounted several
feet away from the turbo, at the end of a very long and thin hose (it
routes near the radiator and the air filter, but you'll have to find
it by tracing it from the manifold - there are any number of thin
black hoses under there).
If this hose falls off, you lose boost. If it blocks (usually owing
to water inside it freezing), then you get over-boost. Try blowing
through it (ideally with some dry air) and checking the connections.
