This could be an issue of fuelling, a lot of American owners (mainly in Golf R's) have a similar issue where the timing is retarded / wont advance on heavy acceleration due to knock caused by poor or low octane fuel. This can bee seen by a dip on the dyno graph.
This is also what happens if you run 95 octane fuel in these engines, they work fine but you'll loose power as the engine ignition timings become limited.