Unfortunately, that's not enough information to make a determination as to your problem.

I personally have never heard of an "E-go" brushless motor, and from the lack of other responses, we can safely assume nobody else has, either. Can you provide a website with the motor's specifications? 3700 RPM only tells us how fast it turns on a single Volt of electricity. It doesn't tell us anything about how much power it can handle, or how it should be used.

What "9-cell NiMH" are you using? If the motor was cutting out, it sounds like you're drawing more Amps from the pack than it can provide. That means the pack is too small, and adding cells will only make the problem worse. How many mAh, what size cells, and where did you get them? Cell capacity, cell size, and cell quality have a huge effect on how the battery reacts to heavy loads, and how much power gets to the motor.