over gearing the motor and excessive arcing will can cause this to happen.

1. check the condition of the brush spring, maybe it has been damaged from excessive heat? if the spring goes soft then u will get brush bounce etc.

2. check the comm make sure it is tru.

3. what brushes are you running? with hotter weather u might have to go to a lower silver content brush like the Fantion 321M

4. consider timing the motor down to reduce arcing and reduce heat.