If I say 9.5v at the beginning of a charge....

I would cringe!!


I would bet that you are going waaay under 9v under load and that is a bad thing.....

10 minutes is a long time on a Trex. I'm guessing the "sputtering" you are talking about is the LVC of the ESC


when I had my Trex setup for 3D (3000 rpm headspeed)
I would stop flying after about 5 minutes.

when set at around 2400rpm, I quit flying at about 8 minutes