Hello,
It is my guess that the hard drive is filling up. Because smaller documents are working fine, we have to assume that the permissions and other materials are working fine too. What is not working well are the bulk documents.
I would guess though that a 6.4 GB drive would be able to handle the print without a problem, as I would guess that your OS install is using about 1/2 of it. Curious problem indeed.
If you are just using this computer as a print server, I am wondering if you would entertain using a different OS instead, such as Linux, which has much less overhead, and is designed to be a server from the get-go.
Christian