The thermal head or motor is too hot.
 

Head:
If the thermal head becomes even hotter, the thermal head may produce imaging on the paper in areas not intended to be printed. This condition is normal when printing documents with a great deal of “black” content.
The printer will stop and resume printing after the print head has cooled down.
 

Motor:
Do not use the printer continuously. If the printer is used continuously, the motor will become too hot. In this case, the printer will stop and resume printing after the motor has cooled down.

To avoid or delay the onset of this condition, print using a lighter density setting, reduce the amount of black space being printed (for example, remove background shading and colors in graphs and presentation documents), and make sure that the printer has adequate ventilation and is not located in an enclosed space.