HP 8600 Officejet Pro will not stop printing gibberish.

My HP Officejet Pro 8600 will not stop printing once it has received a job. The printer continues to turn out page after page with only one line of random alpha-numeric-symbols at the top of the page. The only way to stop the process is to power off the printer. The printer continues to function as a copier and the automatic document feed is functional. The cartridges are fresh HP units. The printer is about seven years old but has seen relatively light use as I am traveling for work most of the year.