This happens fairly often at the club in my area and most common remedies:

1) Check steering servo throw, if over extending can be causing brown out if steering is full lock when crossing the loop
2) Check ESC fan/temps, (re-solder ALL wiring) can be getting brown out @WOT when crossing the loop
3) Check connector to PTX, look for loose wiring/pins, a fresh crimped connector can also fix the issue
4) Check location of PTX, move away from other electronics, especially the motor
5) consider increasing voltge on BEC to 7.4V which improves efficiency and increases power to PTX
6) as a last resort, hook PTX directly to battery by the balance leads