Just for kicks, adjust your steering end points. I had a similiar problem my SCTE (savox servo, rx8, futaba tx/rx) and it was a 2 part problem. Item 1: at full lock (like diving into a corner) the steering linkage was binding a bit, making the servo work harder and pulling more amps leading to a mild brown out. Item 2: the bec in my RX8 was starting to fail, repaired by Tekin.

After each step it got better but didnt always go away until both issues were addressed.