Hello, sounds like a driver issue. What driver are you using? If you are using PCL 6, I would try using PCL 5. I have seen random issues like this where one user could print fine, while another user would print nothing but grayscale. Come to find out they did not push the driver out properly to all the workstations. But I am almost certain the issue you are having is a driver issue. Also, does not hurt to update the firmware.
Hope this helps....