slow network printer - help

I have a network of WinXP PCs, one with a Brother MFC-8500 printer.
Most of the time printing works fine, but every know and then a print
job will take a very long time to print, it will take about 5 minutes
for the first page to print, then another 3-5 minutes inbetween each
page. It seems to not matter what kind of file it is, how many pages,
what time of day, nothing. It just decides to be slow now and then. The
PC that is acting as the print server is usually not doing anything
else, it's set to run windows in the most efficient mode (light on the
display settings), Performance is set to Background services, there are
no extraneous programs or files on it, I run defrag/virus/spyware
programs religiously on all the PCs, and just can't figure out what
else to do to make it go faster. It seems like it just likes to hang
now and then. Any ideas?