Weird issue, I used to run one of these in my sct410 with a very similar motor. Sounds like weak solder joints, but killing a battery that fast is weird.

Can you try a different battery and post some pics of your sodler joints? These are the older mmp? Not the newer style?

Also those mmps have really weak bec, I always ran a external Bec. It seemed to alleviate some of servo problems.