I looked at the website, specs indicate 6 volts to 25 volts. I'm assuming you are using a 6 cell (7.2 volt) battery. You should be OK. Unless something is drawing a huge amount of current.

If you can obtain a meter, hookup wires to the battery and run them outside the tank. Connect them to the meter, run the tank and walk along so you can see the meter. Check the battery voltage when the unit stalls.

Double check all your DIP switch settings also. Make sure you are not in Lithium mode (see manual).

What radio receiver is hooked up?