i would get this checked by a recognised dealership, sounds like its an engine management problem which a Volkwagon dealer will need to look,
i recommend to ask them for full diagnotic check up, they will charge £140 jus to check the whole car, they may also find other problems, but at least you will get a proper answer and know where you are
on the downside the cost may be high, but at least it will be fixed..
goodluck