The comms are never perfectly round, even from the factory.

Arcing will always occur to a certain extent, it's damn near impossible to get a motor to operate without a little bit of arcing. The brushes will always bounce a little bit. The Fireball's are a bit on the "inexpensive" side, and they may be a little tweaked from the factory. Just get the arcing down to a reasonable amount and see if your glitching goes away. Phatkat has given you just about all the info you need to get rid of your arcing problem.