Your buggy is brushless. The speed control (what has the fan on it) is setup for LIPO battery use. A lipo battery (unlike the stock NIMH battery) cannot be fully drained. If that were to happen, it could not be recharged. Thus, the speed control senses battery voltage, and when it drops below 6v the ESC cuts power to the motor...first down to 50% and then to 0. But you will still have the fan running and the steering working.
The manual should show how to disable this function (function is called Low Voltage Detection, or Low Voltage Cutoff)
But it is a pain to adjust the speed controls features without the optional program box. (can be had on ebay for $15)

A fully charged nimh battery should fix your issue. It will not hurt the NIMH battery for the LVD to be on. It will just cut power to the motor when it seems like you have plenty of power left in the battery. In reality, even if you turn off the LVD you will only gain about 1 minute more of runtime. When the nimh battery hits 6v, it doesn't have much juice left in it.

If you like your tornado now, wait until you upgrade to a 2s lipo battery. You can double (or even triple) your runtime and gain acceleration and 5mph more top speed.

The other Port on your radio is cool, you can get a cable (or it may have come with the buggy) to plug the radio into a USB port on your computer. This will let you use the radio as a control for RC sims like VRC pro (of which there is a free to play version)