Sounds to me like the groove that the circlip fits in is worn away, mine did this and the circlip went inwards allowing the sprocket to follow and grind into the swingarm - it's fairly common I'm told.
http://mysite.wanadoo-members.co.uk/...14666547906022
I bought a new carrier and they have changed the design, there is no longer a circlip in a groove on the carrier but a 2 piece carrier with a solid alloy piece inplace of the circlip part.
http://mysite.wanadoo-members.co.uk/...91122518105882
Edited to ask; why don't the pics show on the post, yet the links work?