I just noticed that out of the 3 copies of the drivers, two are pretty much the same and implement the ENC28J60 Rev.B4 errata... Not Rev. B5, which is the same as my chip, if I read EREVID – and seems to be anything manufactured >= 2006. Had you guys worked these out or where you running on version 4? Because there are a few more erratas in rev 5 for Rx/Tx and mostly in Half Duplex sending!
EDIT: As far as I can see, errata #12 and #13, which both affect transmission have not been fixed yet.
EDIT++: Sorry I forgot the link http://ww1.microchip.com/downloads/en/DeviceDoc/80264d.pdf.