RE: F3A Formosa ARF Park Flyer GWS
Yea it's your speed control setting, I use castle ESC's as well and they are great. I think the solution for you is to either get a bigger battery, change prop size or change the auto cutoff feature on the ESC. What is happening is the current draw past 3/4, is drawing to much power and your BEC on the ESC for the reciever gets to low so it cuts off power to the motor. You can disable this feature but then you risk lowering your voltage to the point that the servos and receiver don't function as they should.
The best fixes are to get a higher current battery if you like that prop.. Like a 2100 or better.. Or drop the prop size of course you give up thrust when you do that, but a 10x4.7 or a 9x6 should still give you good vertical with that setup.
I went with a much lighter and less current hungrey brushless setup so I could run a lighter 1500 pack. I have found the lighter the better with these electrics.. Sure you can put a larger engine but then it takes more battery, so in the end all you do is add wing loading. With mine I can soar with power off and land soft, because with light wing loading a plane will always float better.