Cause:

This occurs if the toner wax remaining on the fusing belt is uneven.

This may occur if:

Paper with a thickness equivalent to Paper Weight 7 or 8 is used, so that the required temperature is high
Slippery paper is used
An image covering a wide area is printed

To adjust the following settings, pre-register the type of paper in use as a custom paper. For details about registering custom papers, see "Registering a Custom Paper", Paper Settings.