Pity I didn't spot this thread sooner as I had the same issue with the slipperential in a Savage and solved it the same way
http://www.rc-monster.com/forum/showthread.php?t=27208
One thing note is to make sure the overlap points in the 3 wave springs are evenly spaced out when you put them on. This seemed to be part of the issue with just two as when they were really tightened down it seemed to create uneven pressure on the slipper.