(T) Diff problem??

Hi all. My sisters 1.2 8v mkII punto has a terrible habit. It behaves as if the clutch is slipping, but i've been told by a fiat main dealer technician that sometimes a prob in the diff causes this behavior. If you put the handbrake on, engage first and let the clutch out, the car will stall, so the clutch is biting. (incidentally its a brand new clutch in it).
However, when you are driving around, if you put your foot down in it it revs away, sounding like a slipping clutch. i've been told by this fiat tech that one of the wheels in the diff (crown or pinion, i don't know for sure) is "sweated" on, and this sometimes starts turning on the shaft under load causing the loss of drive and a clutch slip like noise and symptom.
he says the official dealer fix for this is to spot weld the gear to the shaft stopping the slip. Have any of you heard of this before, and do i need to split the 'box to do it, or can i see the shaft and gear when the box is out of the car and weld it with the box asssembed??