I've been having similar problems with my differential. I just went to an aluminum differential.

One thing to make sure and do is when you assemble the differential and install it into the transmission, make sure that the adjuster screw is on the same side as the slipper clutch. If not, your differential can loosen up on you when you start applying power to the wheels. I got this advice from the LHS today.

I'm guessing this is one reason why my differential has been coming loose on me. If anything I won't be melting differential gears anymore.