I think I finally figured out why my buggy has been running so hot. there was a good deal of binding in the center diff mount, and it caused the spur gear to rub the motor mount. I put a small shim on the spur gear side of the diff (same ones I use to shim the rear diff), and I didn't tighten the bottom mounting screws (plastic side) of the diff mount 100% (backed them out about 1/4 of a turn)and that solved the problems. The drive train is so much freer now, it's amazing. I'm not sure why was binding in the first place, perhaps the motor mount is bent ever so slightly.
If that doesn't help I put on the big a$$ losi heatsink too!