I would first make sure that the paper type in the driver matches exactly to what is loaded in the the tray, size, type, and orientation.
I have a new Phaser 6510.
Am I correct in assuming that my Xerox Color Impressions white 120gsm paper requires the paper type setting set to LWCardStock?
If so, I get a numeric error every time I set it to this type or the machine asks me to check paper type.
It prints okay on plain paper type setting, but large dense colour blocks on the paper tend to break up or a very patchy.