Sounds like a brownout, where the power supply to the pixhawk dips and causes a reset. More details about the hardware such as photo and a wiring diagram of the system might help including where the pixhawk gets its power, if you can do a sketch.
I am flying with U7 490KV T-Motors fitted with 20x5.5inch props. On Monday when i did the first flight i took off in STABILIZE mode and when i switch to ALT HOLD the motors stopped and sounded like Pixhawk was rebooting. Tuesday we placed the HEX on a test bench with the props and when i open the throttle and pass 50% the motors stop and Pixhawk resets again. i checked the 30V power module as i am flying with 22000mAh, 22,2V 6S battery and found that the solder on the positve wire coming from the battery was loose. this i fixed.
Today i did the same test and again the motors stopped. fitted 18x6 props and not stoppage beyond 50%. only have a tilt issue now
Sounds like junk getting from the power supply to the PixHawk, but without seeing the details of the system, it is impossible to say anything definite. Look for ground loops , make sure that pixhawk is only connected to ground at one point ( e.g use Opto ESC's etc)