Do you still have the original diff washers and balls fitted?, also are you using the original bronze bushes in the centre of the diff & outdrive or replaced with bearings?
I found that if I fitted 2xbearings in the centre of the diff they were very slightly to wide,meaning the diff wouldn't tighten fully, now use 1x bearing and 1x bush in the centre of the diff,works fine, I am also using the kevlar diff,1xmetal idler and 1x nylon idler and AE spur. I'm not sure on the diff balls, I never fitted the kit ones, put some others in that I had in a parts box.I also started to use AE diff plates, direct fit, I have had no probs with these, but others have so take care! if you use them
Been running for around 3xmeeting with a 6.5 and all is well so far

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Dave
www.norfolkbuggyclub.moonfruit.com
kyosho optima,Bosscat,Boomerang,,RB5 Vega,RB7,,RC12 5.2, TLR22-4,MP9,HB807T
Flask of tea & a rollup
Anglia model centre & CT Models
http://www.oople.com/forums/showthread.php?t=28117