Why did my diff self destruct - again!

[RC944]

ok so im bashing the b3 after having rebuilt the diff as per intstructions and slowly but surely the car starts losing speed and finally all i hear is the clutch slipping but no drive transferred to the wheels.... fearing the worst i open the tranny case to find (for the second time) that the diff gear with the bearings has killed itself - what was once a single plastic gear has now become two pieces with the plastic on the inners side of the bearings separating itself into a separate piece of plastic that spins freely from the geared part on the outside with the bearings acting as the roller! given that this is the second time it has happened and each time i need to wait 2 week to get a new diff gear im hoping someone can point me in the right direction! to my mind there could be a few causes:

- overtightening of the diff washers via the diff bolt
- left outdrive and right outdrive wrong way around in tranny case
- problem with the left outdrive hub that has diff spring to maintain correct tension with the diff bolt
- anything else?

im convinced that i have re-assembled the diff correctly but am really struggling to find a cause!