arcing = burnt comms - too much timing, driving style (braking, pumping the throttle) overgearing, out of round comm, poor spring tension causes arcing which may lead to overheating. if you overheat too much it burns the brushes which damages the carbon molecules. on some cases it will overheat too much and cause the brush ends to crystalize.
you can deglaze the brushes by cleaning it with a comm stick. if it is too silvery or worse bluish it must be replaced.
you can get away with slightly bruned brushes on small races and practice runs. BUT on big races you must change it and use a new pair.
REMEMBER when you use brand new brushes ALWAYS CUT THE COMM.
one more thing that you might be overlooking is that your springs may be too soft or worn out try to replace new springs. or your timing might be too high.
this is just my 2 cents. im not trying to say the other posts are wrong. just wanna help out. who knows the answer might be 2 posts combined.