Hi there, I'm not sure if much of this will make any sense, but may be of some help...
I have just bought a mk5 GTi from an independent dealer, he said he had the car in as a part exchange and was now selling it (as they do).
He was upfront and honest with me, and told me that when he took the car in it was misfiring and and so he replaced plugs, leads, and coil pack but still didn't work. He then changed the fuel pump, and apparently it was this that was causing the problem. Might be worth looking at?
the car is under warranty and has the ultimate warranty cover with autoprotect so if I get any other problems, I will be on the phone to them!