this was happening to me a few years ago, but it would only jump out of reverse
if i rev'd it high, ie reversing up a hill, after a few months i had no reverse at all and all the forward gears was making one hell of a noise...
contacted VW for a price on a gearbox and they said £1100
so i just went and picked one up from a breakers yard... £250.
if i had it looked at when it started to jump out of reverse then maybe it could have been saved, try and get it looked at before its to late