HP CLJet 5M keeps restarting print jobs

I have a HP Color LaserJet 5M attached to an HP JetDirect 300x on a Windows NT 4 Ethernet LAN. Larger print jobs (Over 7 pages of Word/Excel documents with colour) will start printing and reach page 6, for example. Then the printer will restart the job from page 1 again….I suspect there is some hiccup between printer and print server and have tried several options – various drivers (PCL, PScript, HPCLxxx), even setting High Speed = No in the printer Parallel menu. Help!