I've had the same on mine, where the circlip groove wears and it all starts backing out.
You can replace the sprocket flange with the later design which replaces the circlip retaining method for a spacer instead.
Cost from Ducati is around £105 for the sprocket flange and spacer, or have a phone around the breakers as I got mine for 30 quid.
Don't know if this is definitely what it is though - I had loads of swarf on the wheel and hub when it first started happening on mine. More info here:
http://ducatisportingclub.com/contro...d.php?tid=6972 Check your chain for tight spots and also check the rear sprocket as well - the chain might be catching if the wheel bearings or hub is worn and it's gone out of alignment.
Other than that I'd pull the assembly apart and check/clean everything before reassembling and checking it again.
Good luck!