it sounds like you are pulling to many amps for your speed control. The reason it doesnt do it with a 2s pack is because you are giving the motor only 7.4v compaired to the 3s 11.1v and you are not pulling as many amps with the lesser amount of voltage. If you want to get more power than your 2c, your gunna have to get a pheonix 25 speed control and use your 3s pack. It also all depends on were you are going to be flying the most, if you mainly fly indoors, i would stay with the 2 cell, but if you decide to venture outdoors, i would recommend the extra weight and power of the 3 cell with the pheonix 25.

Someone please correct me if im wrong, that just seems to be the problem too me...

Good luck