12 to 20 minutes means an average of 3-5C, which on the surface is well within the capability of TP2100s. The real issue here is maximum current draw, and how much you used it. IIRC, TP2100 packs have a rating of 6C, or about 12 Amps. If you have a power system that draws more than that, you could be damaging the cells.

Two questions that might clear things up for you:

1. Was the pack that's experiencing poor performance in the plane when the gearbox was binding?
2. What was the second question I was going to ask you? (my train of thought derailed)

The binding gearbox could double the amount of current being drawn by the motor.