Even though it sounds fixed, I thought I'd relate a story. I had a similar thing happen to my car recently, though I thought it was a little underpower. After trying everthing without satisfactory results, I remembered my radio is futaba and I needed to reverse the throttle channel
so when I thought I was braking, the esc was actually going full reverse to positive throttle, something they arent usually designed to do as smoothly as the regular kind of braking :p
Needless to say it fixed the problem, and since reverse was only set to 50% in castle link, I also gained a shiteload of power haha.