i had a similar problem on my driver, turned out to be a fairly small hose at the back of the block had split underneath. its located at the back of the block on the right handside,its only abt two inches long. check there and see if thats ok.
car will still drive because ur pushing the air out the split as soon as u stop accelerating the air comes back into the split causing to much air in the system upsetting the air and fuel mixture.
well thats wot i was told on mine but bear in mind mine aint a gti