If you're using a 3S lithium battery, the problem is in your cutoff voltage. As previously stated, the cutoff for a 3S pack should be between 9 and 10 volts. Also, as previously stated, your pack probably can't dish out enough voltage. With the MX400 and the T-Rex helis, I think it's best to shoot for a pack that can discharge constantly at least 18 amps. 20 or more is preferable.

I highly recommend you get a wattmeter too. Or, even get the Hyperion E-Meter, which is a digital voltmeter, tachomter, ammeter and watt meter that you can connect between your battery pack and ESC and use to determine exactly what figures your power system is pulling. You'll definately save in the long run with knowing what amperage the power system is drawing (worst case situation: you're doing a very hard climb-out which causes a big amperage spike in mid-flight, battery conks out and heli crashes! UGLY!). The more you know about your heli on the bench, the more confident you will be in the air knowing that your setup can take whatever comes to it.