RE: eflites - ascent glider
You're putting too much power through the motor. The propeller/motor combination was designed for 8 NiCd/NiMH cells. You're pumping the equivalent of 10 cells through it. 25% increase... No big deal, right? Wrong. A 25% increase in Volts brings a proportional 25% increase in Amps along with it for a 56% overall increase in power. Unless you reduce prop size and get the Amps down, the motor won't last very long. Even then, the excessive RPMs caused by the high voltage will signifigantly reduce the lifespan of the motor.
Running at partial throttle is not a solution, unfortunately. ESCs are just really fast on/off switches. They put full-power pulses through the motor many thousands of times per second. These full-power pulses will still get the motor hot, because the motor simply can't take the stress...