Ubuntu Linux

My server: Ubuntu 8.04 / 100 Mbits/s Ethernet / 240GB Sata HD / NFS server for the users of the workstations
The workstations in my classroom: 20 brand new mixed Ubuntu 8.10 / 9.04 Dual Core 100 Mbits/s LDAP authentification
I have more than 1000 possible users (all the students of the school). Their files (and .profiles) are saved on the NFS server. They can work from any workstation in the school with their personnal username and pwd.

My problem:
A standalone workstation with local authenfication and local user files works perfectly good, fully reactive.
The problem arises when 10 -> 20 users log at the begin of a lesson on the LDAP/NFS server.
The server‘s HD works continuously, sometimes more than one hour; the workstations are very, very slow: totally unusable, sometimes freezing during about 2 minutes. After one hour hyper-activity of the server‘s HD (and revolution among the students), the users get progressively normal service of their workstations. All their files are normally accessible on the NFS server.

The "top" command just show a few % CPU activity on the server.
The Desktop Visual Effects have been disabled on the workstations.
Tracker has been disabled on the workstations (I read on this forum that Beagle could be problematic).

The server software was renewed in december 2008. Ubuntu 8.04 took the place of an old Fedora 5 distribution. Old PIV 382 Mo RAM workstations were replaced by the DualCore stations.
Nothing has been changed on the network.
Before the renewal, I’ve never noticed that slowness problem.

My questions:
1. Do someone here successfully use a similar configuration ?
2. What can I still do to make the workstations usable in september, when the students come back (our present "solution" is to use local users on the stations with a "on-demand" Samba connexion to the NFS server).

Any answer really appreciated.