Lint-o-rama
The dryer needs to be taken apart and cleaned out. Burning on the heater box happens when the airflow gets restricted--sometimes due to restriction in the front channel of the drum behind the lint filter. Mine was down to a quarter-sized hole through which all of the air was attempting to pass. Ralph helped tear the whole thing apart, and we vacuumed about ten pounds of lint out of the works and the cabinet.
Or, it could be on a crummy vent line. But again, lint will build in the ducts if the case.