The Tamiya style plugs have a higher resistance that other types of plug like deans or corraly connectors. i have melted some together in the past when using a stock motor.
It can also happen when the connector inside the plug is arcing which can cause the plug to get scorched on the inside, i had this happen on a charger i have.
to fix this replace the tampiya style plugs with Deans plugs or some other after market low resistance plug.

you may also want to check to see if the motor is over geared as this will cause the ESC, Motor and battery to over heat. try lowering the gearing ans see if that helps.