the phonix 25 is probably set to hi on the voltage cut off, if it is set at 8 Volts or higher on the nimh batteries it will cut off because the battery drops volts fast and then back up but it is enough to shut the throttle off, try setting the volt cut off down to about 4 volts since you are using nimh batteries, on lipolys the setting is 6 volt for a 2 pack and 9 volts for the 3 cell lipoly.
your motor, 3700 KV times I will say 8 volts the batteries you have 9 cells - 8x3700= 29000 divided by gear ratio 4.43- 29000=6681 rpm at the prop, that is pretty slow for the props and pitch you gave, the smaller and lower prop size and pitch need a lot more rpm than that, just a guess put a 9070 or 1047 on it and it should improve a lot. the best thing you can do is use 3 cell lipolys, they are 11.1 volts times 3700 KV =41070 divided by the gear box 4.43 - 9270 rpm at the prop which would amke the small props a lot more efective. .