My opinion most want listen to.
Throw the damn thing away as it is nothing short of a fire hazard. There has never been one that is designed to be used on a 15 amp circuit or one designed to be used for an extended length of time.
This is a resistive load of 12.5 amps at 125%. Now you have a heater that is 1875 watts or that little heater needs a circuit that can deliver 20 amps but this will not stop the heating of the cord.