The MMP has a pretty weak BEC in it so it may not be totally the servo's fault. I don't know why, I guess they thought it would be put in lighter 1/10 vehicles that didn't need lots of power for the servo. Guys who were putting the MMP in 1/8 buggies we're finding this out the hard way too, for a crawler type vehicle you definitely want an external BEC.
I'm pretty sure Castle's default voltage setting for their BECs (internal and external) is 5.1v to avoid blowing out any vintage 4.8v rated servos.