"A 7-cell NiMH?" You'll need to be a bit more specific. The reason I say this is because my first instinct is to suspect the battery. What brand? What capacity?

There are many things that can go wrong with the battery. You could be charging the battery too fast, which would cause it to not take a full charge. A battery too small to handle the current being drawn by the motor will sustain serious damage after a couple of flights. There's always "infant mortality," where one or more cells fail when the pack is still fairly new.

Charge the pack up on a good peak-detect charger at a rate no more than the battery's capacity in mAh. 700mAh pack should be charged at .7 Amps, for example. Measure voltage hot off the charger. Let it sit overnight. Measure the voltage again. It should drop some, but not below about 8.8 Volts or so.

The other possibility is that the motor has gone south. Remeber that these are mass-produced motors that we're pushing far beyond their original design limits. They're meant to run a hair dryer fan at 6 Volts and 2 Amps, not fly an airplane at 9.6 Volts and 10 Amps. You're going to see an occasional early failure.