Hello, not sure if you have tried this but try updating the firmware see if it changes anything. Also, is it possible to try using a PCL5 driver? From my experience, I have seen the PCL6 have more issues than the PCL5, your issue sounds like a driver issue in my opinion. Reason being is you said when you do internal test prints it prints fine. Also, what applications are you printing from? I would check those setting as well, and see if the issue is only happening with a certain application or is it from all applications.
Hope this helps....