Hi, I am Steve, lets get you some help.
Typically this indicates that the printer needs cleaning. However, the fact it is only happening on Legal Paper suggests it is specific to this type of paper only. I would check that and perform a clean of the printer just to exclude this as the cause.
The first thing to check is that the printer is configured for Legal Paper from the paper tray supplying said paper.
Click here for the user guide with instruction on how to set paper size.
If the printer is set correctly then you need to make sure that the application(s) you are printing from a setup correctly (specifics will vary based on the application) but it needs to be setup for legal pager size and to be accessing the correct tray in the printer.
If none of these work, I recommend installing the printer driver again on the PC.
Click here for a list of drivers fro this printer.
Please let me know how you get on.