Just a wild guess, but from what I see, I'd say the brush springs were too soft, letting brush bounce create extra heat. Not enough to "harm", but enough to affect performance. Assuming you were running offroad, there is a lot of extra vibration and bouncing going on -- brush bounce can quickly over heat a comm. The color change on the comm and brushes clearly show heat, but your gearing doesn't sound out of range at all -- I've run that gearing range locally offroad with no problems at all, but with heavy springs (like Red/Red or Purple/Red)
"If you cannot win, make the one ahead of you break the record."
Biff Racing Team #420 (Ah... The "good old days"...)
The local indoor Offroad Track: The RC Race Barn
TLR22-3.0, TLR22T-2.0, TLR22SCT-1.0