Originally Posted by GerryH

Is the buggy always fine when you start running it? After how long of driving does it start acting up?

Do you have your end points on your radio set correctly? If you're going lock to lock, the servo can pull a lot of power when it stalls. That servo pulls 4.2A at lock/stall and the BEC in that ESC is only 3A.

As Hitokiri mentioned, you'll probably need a external BEC.