I have been racing mugen for over 2 years. The gears last forever, I still have the original set. And you want a very small click of play. I listened to someone on here who said shim slightly tight, grease and break in. Well it starting doing what you described. Lucky for me I break my stuff down often and caught it. Shim you ring and pinion like you would your spur/bell. Also, take the top of the diff case and lightly sand it flat on a table to make sure it is even and flat.
Then check the evenness by setting your diff on the center diff mount and spin it. It will expose any unevenness no matter how small. Mugen gears are bullet proof if treated and maintained right.
ETA, not quite as loose as the spur/bell, just a hair tighter. I shoot for smallest amount of play, but there is still a "click". If your ring gear is even all around and your mesh is right, it is almost impossible to strip them
Hope this helps
Last edited by dreaux; 04-18-2011 at 12:24 PM.