I have a heavy copter I am building with T-Motor U8 (environmentally sealed versions for bad weather) and a T-Motor 70APro ESC connected up to 2*5S (aka 10S) battery supply.
My first problem was I could not get them to work with the Pixhawk until I realised my obvious mistake - they are optic isolated ESC so no power. OK Fixed that only to find problem 2 - Sync problems. Lots of practice an research lead me to the 5V bec. I was supplying only a 3.3V BEC and moved to a 5V 2A DC/DC converter and capacitor for very clean supply. They only draw about 100mA all up so seems good now. No more sync problems - I think. These problem did not appear directly off the RC Receiver, only Pixhawk - but I think they were reasonable problems one I realised my mistake.
Which leads me to my last problem, which could be a form of Sync problem but it appears not just with the pixhawk but also directly off the RC Receiver. ESC have been calibrated.
Short version - Power up slowly between 0-50% and, although spin up is slow/gradual, continues correctly to gain power. Then from about 55% throttle power drops until it gets to 90+% throttle which then give me full power (more than the 50% mark anyway).
Feels very much like a sync problem, but nothing to do with Pixhawk.
Any suggestions? What other information is useful to help?