That reminds me of a box failure I had in a Fulvia - big bang, stop, roll car, all OK, drive a bit, still OK, then big bang again. Turned out one of the gears shed some teeth and all was fine as long as the bits were at the bottom of the box, but they kept getting picked up and...well, you can guess the rest...

As Craig says, get that diff out pronto before any loose bits take the rest of the box out!! Don't know if you can have a good look round in the casing with the diff off to check for debris?

Good luck and let us know...