I'm part of a team that's been working on a Multiplex EasyStar, and the motor we've been using is an E-flite 400h 4200Kv brushless. Today, while going through our pre-flight checklist, we found that the motor cut off just before full throttle. We were able to throttle up again once the buzzer on the motor played a few tones. We decided to fly around 90% throttle and see how long we could go. We had about 10-15 seconds of powered flight before the motor failed again. We thought it might be the battery, since we were running with a 2S lipo, so we switched to a 3S. This time, 100% throttle was attained, but it lasted for only about 15 seconds again before dying. During each flight, control of the throttle wasn't regained after it became unresponsive, but the control surfaces remained responsive.
Following the flight with the 3S battery, the tones played quite a while after the landing, signaling a ready motor. We then pushed it to full throttle while we held in place, and the motor lasted for 5 seconds. When the tones sounded again, we opened up full throttle with a duration of 3 seconds. Since the control surfaces can respond even while the motor is out of commission, we think our problem isn't loss of communication with the transmitter. Instead, it seems to be linked to exceeding a max temperature parameter. We would appreciate some tips on troubleshooting this problem.