First of all, your brush springs may be too light. You are getting brush bounce.
Also, I would suggest only breaking in the motor at 2-2.5 volts for about 300 seconds (5 minutes) Any more than that is wasting the brushes.
Sounds like you are running the motor for 15 mintues of break-in. That is too much. Especially the 5 volt break in. Are you using a fan? If not, things could be getting too hot, and if you are, that is a pretty significant load to just break in brushes.
Try 2-2.5 volts for no more that 5-6 mintues. Use purple springs too.