Assuming the correct stock settings don't work, you can just "roll your own" as it were.
The following is cut and paste from something I documented previously, so pretend it doesn't mention Linen, just replace it with 13*19 @ 300GSM. (Cause I don't have the time to take pictures of the UI with this particular scenario)
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Go to the copier and log in as the admin.
Then go to Machine Status > Tools > Common Service Settings > Paper Tray Settings > Custom Paper Settings
Choose one and give it a name, and adjust the Adjust Image Transfer setting
Usually the percentage needs to be raised, I suggest going in increments of 10% for both sides until you get a number that works with your stock
Then load a tray and choose the created Paper type from the list. (I called mine Linen in the above steps)
Then, if the Fiery/Freeflow gives issues, you will have to choose the stock type as Use Machine Settings (Fiery) or Use Loaded (FFPS) since the actual stock name won't exist in either of them.
As for actually fixing the printer instead of a workaround, that would require a service call from a professional.