nfs server load goes high when users login

let me exaplain the setup first. im also not sure if this bug is due to ubuntu machines or not.

i have a nfs server. we are using it since 3 years atleast.
cat /proc/version
Linux version 2.6.18-1.2798.fc6 (<email address hidden>) (gcc version 4.1.1 20061011 (Red Hat 4.1.1-30)) #1 SMP Mon Oct 16 14:39:22 EDT 2006

we have a lab of 75 *ubuntu* machines. home directories of users are mounted on this nfs server. so whenever a user logs in, a nfsd process is opened in nfs server.
Recently, after we switched to ubuntu machines (till last months 75 machines had debian), the following problem has arised. (though i dont think, its due to ubuntu. since we had observed such problem once or twice last year also)

As 40-45 users simultaneously log in to the lab machines, load on this nfs server goes high around 20-25. and then user login hangs or becomes extremely slow. and no user is able to do any work.

we have raid0 on nfs server. kjournald process seems to be on top when we do 'top' command, though cpu load is very low around 20%.

In /etc/init.d/nfs file, i changed the value of RPCNFSDCOUNT to 20 (which was originally 8, thinking this might solve the problem). and now i have again increased it to 50. but still problem persists.

$ps -eaf | grep nfsd | wc -l
52

is any more info needed?
can any of you guys help me?