It's a 13 turn motor. I am running it most of time on 4 cells only so that's pretty mild compared to 6 cell load.
No the motor never overheated, because I am running pretty conservative pinion.
I don't think it's a shorted arm since the motor was fine for first 5 runs and just after it started show these symptoms. Then I had the com cut and changed the brushes. Got about 5 runs and here we go again.
I do realize that mods require quite a lot maintainance, so I would expect a power dropping the more runs it has in, but I don't think it should stall like that. Also the fact of 4 cells should make it a lot easier on the motor.