When I see the words "brown out" and "Savox" in the same post, regardless of whatever else has already been tried, a different, non-Savox servo is what I'd start looking to try next even if you've already gone overkill with a whole external BEC.
That being said, cogging off a hard punch of the throttle, especially in a 4wd SC, is often a symptom the battery itself just not being able to provide enough power for that much motor. Brand, age, spec of your lipos?