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The 'overboost' problem is very common on VE engine direct injection TDI's.

The symptom is the boost cutting out under load, up hills or at motorway speeds, generally in third gear or higher at around 3000rpm. If you cycle the ignition it resets it.

There are a few main causes for this problem:

The main suspect in 90% of these cases is the 'hot film' Mass Airflow Meter, especially Bosch Maf's which degrade or fail a lot earlier than the Pierburg type.
Running the engine with the MAF disconnected will cause the ECU to use default values, if the car runs better your MAF is totaly knackered, but this method will not allways diagnose a worn or failing MAF.
First try cleaning the element with a suitable spray solvent, and disconnecting and reconnecting the connector several times.

The EGR valve could be gummed up and not closing properly, causing hot oxygen poor exhaust gas to interfere with your fuel/air ratio. If your EGR valve is on its way out, or in need of cleaning it may start to make a 'squawking noise' like a goose. Solved by cleaning or replacing your EGR valve.

If you have a 90bhp model the waste-gate may be fouled or sticking.

If you have a 110 the VNT actuator rod & lever may need a bit of WD40 spraying on it, or the VNT vanes may be sticking in the 'full boost' position, this can sometimes be remedied by aggresive use of the accelerator, or dismantle the turbo and clean the VNT mechanism.

Inlet Manifold fouled, due to the EGR system the inlet manifold is prone to developing a thick layer of sooty crud, which can seriously reduce the volume and airflow of the manifold, this layer can become so thick that it alters the reported manifold pressure.
Remove manifold and clean.

Boost Leak or Boost control leak, check all boost side pipes and hoses especially to and from the intercooler, hose from n75 to the vnt actuator or wastegate, and the small tube to the MAP (on AFN engine with pressure sensor in the ECU check or replace this tube, it is prone to leaks and as mentioned above it is badly designed and traps liquid.)
Feel all the braided rubber hoses where they join other tubes or touch anything, as the rubber can wear away inside leaving the external braid looking intact.
It is recommended to just replace all the small tubes in the control system rather than try and find a minute leak, they cost next to nothing.

Timing can cause the same overboost symptoms. If you are overdue for a timing belt change or sometimes even if you have recently had one, your timing may be too far retarded, causing under-fueling.
A worn/dodgy fuel temp sensor can also cause timing to fluctuate.
If your overboost problem goes away for a few weeks after towing, or if a tuning box stops it, your problem could well be retarded timing.
Use VAG-COM/VAGSCOPE etc to check.
Also if you have your radio code, try disconnecting your battery for 15mins and go for a spin and see if there is any change.

Chipped cars/tuning boxes can also interfere with engine management causing the same kind of problems.