That will be a MK3, you need to know how the fuel is delivered in order to start fault finding, I.E is it CFI or EFI? That age I certainly don't think it will carb fed as the early MK3 were CFI.

In any case it sounds like the base idle may be to low hence not being able to substain an idle, however as I say you need to know the above in order to make a start.

Martin