I always found the diff on my 210 difficult to adjust in the car after rebuilding it, like you say it would behave perfectly in my hand (no slipping when holding the outdrives) and then slip when installed in the car needing further adjustment.
You need to break it in on the bench and then drive it gently for a bit whilst adjusting it a little at a time, have a look here:
http://www.rctech.net/forum/electric...er-clutch.html
Once you've taken your time over this the diff will be excellent, I use tungsten carbide balls and a Schumacher U1954 thrust bearing (same size but much better quality than the TD thrust race) and the diff action is lovely
Also on the DEX210 the anodizing wears off the slipper plates and this can throw out your adjustment because the pads then get full of aluminium dust so change these and the pads if that's happened. Hope this helps
