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Now, on my old car, this would have been due to a throttle position sensor going faulty. So, as the Polo GTI does not have such a device, could it be the connection to the throttle potentiometer? I'd jiggle a few connectors around when it's running (no shirt sleeves) and see if you can replicate it.
Maybe it's water in a connector shorting it out? If it's wet mizzle fine rain where you are, then this has caused loads of cars to breakdown in my area today!