Did this problem occurr immediately you got it back from the garage? It sounds to me like a wire has been switched round or something like that, definitely electrical i would say. I have not had anything like this with any of the 9 vw golfs i've owned, two mk3 gti's at present. Electrics are generally very good on these and don't fail bizarrely like this for nothing. If your garage are half decent they will check this out for you for nothing seeing as this has occurred after this work has taken place.