printer problems

Hi,
I run a PDC with Windows NT 4.0, which contains about 25 HP printers, (HP 8000 series, HP 5 series, and HP 4000 series)running with HP Jetdirect print server cards. Everything works fine, except for once every three or four days, none of the printers will print except for the 8000 series. I have to stop the spooler service, and restart it. This works, but it is a temporary fix. What could be causing this problem, and how can it be resolved?
Thanks in Advance..