CPU hits the limit, and causes the network slowdown to become unuseable

Hi All School: Running 2 x xserves both 2 x quad cores running 10.5.6, connected to a promise raid via fibre channels, for storage of network home folders. one being an open directory Master, the other a replica. aprox 290 Clients are imacs running 10.5.4 and emacs running 10.4.11. This has been running fine. We upgraded 16 of the imacs to 10.5.6 when it came out, and we experienced a major kernel Panic when networked accounts logged out. We downgraded them back to 10.5.4 Last week we bought the new "entry level" imacs, which wouldn't boot with 10.5.4, so we performed an archive and install of 10.5.6. (maybe not the best way, but time was precious) The room has 30 machines. When pupils now log on, the network grinds to a halt, the CPU's on both servers, maxs out at 100% (when previously it hardly ever gets near 20%) we turn the room off and all the other rooms log in.. goes back to normal... We had a very bad start at the beginning of september, when we were sold a raid box and xsan to work with home folders.. which didn't, leaving us we now working machines for 3 weeks, with a new head that is not into Macs.. this is giving him fuel to get rid of them (not also because the price of this room shot up considerably, because of the new specs) A new image has been started from scratch, but who's to say it won't happen again come monday morning...but someone MUST know what is happening here.. also, on activity monitor - afp is stating 202 threads.. regardless of how many users... Oh, and spotlight has caused major problems in the past and this is disabled across all machines....ANY help would be appreciated I have spoken to two major apple dealers in the UK.. still no answer... Thanks Nicola Clarke