Which BEC do I need??

[T-Bro]

I'm trying to get this truck running right with my new NEU motor. I put my new 6s lipo 25c 5000mah pack in it with my schuze 18.97kw esc and it's not acting right at all. It seems to work better with the 4s lipo pack. The only other upgrade I've made was a
Futaba S9156 Digital High-Torque Air Servo for the steering. I'm running it with an external bec which is 5/6amps. I just read an article from schulze speaking on how the servos shouldn't demand more than the bec or something like that so it caught my attention.
With the 6s it will run normal, then just go full throttle and flip over without me hitting the throttle. I'm wondering if I am running too much for that bec to handle? 2 servos including 1 high torque which I'm sure draws a lot of current, 2 fans, esc, etc. Do you think I need a airplane bec made to handle several servos to handle this big load, or is it possibly something else that is making my truck go crazy all of a sudden? It never did this until I added this new servo and tried it with a 6s pack.