An old but unsolved Problem: NFS too slow!

Hello Everybody

we have a server, AMD opteron(tm) Processor 852(4 cpu), 16 gig Ram and more than 700 Gig. SATA disk drive +Raid controller.
The Linux is Debian/Sarge with kernel :2.6.16.14-amd64-smp-2006-05-08-hk.

The Problem:

NFS transfer rate over network is terribly slow, by write is less than 1,5 mB/s!!!(read avg. about 30 mB/s) We have checked the paramteres by export on server and mount by clients out and they should be ok and not problematic. Locally the Server has a very good throughput but over NFS is too slow, that we can not use it as a NFS-Server. :???:
Exports on Server:

/test/ client(rw,sync,no_root_squash,no_subtree_check)

Mount on Client:

mount -t nfs server:/test folder -o rsize=32867,wsize=32867

We had tested a lot of various settings on the server and client, but there is no better results. Its throughput is only really better if we use async instead of sync. I know, that there were some problems with nfs and sync, but i unfortunately could not find a solution in forumes and dikscussions.

An help and suggestion would be greatly appreciated. :)

Thx.
Danesh