During a pre-sales call with a tech at HP, I seem to remember him saying that they have noticed a problem with USB to Mac setups - and he generally suggested that we set it up with ethernet. If I remember correctly, I asked about a speed advantage with a direct USB 2.0 connection, he replied that they have seen problems with USB connection and that there isn't a speed advantage versus ethernet.
Also, I think that I noticed in the troubleshooting part of the user manual, that there was information about settings that could result in printing stopping part-way through. It might be a 'time out' setting. I don't have the PDF handy - could this be part of the issue?