From: "Stephan Uphoff"

> Some NICs can not handle a 32K package train.
> This is a well known problem with UDP.(1)
> Recompile your kernel with a smaller default NFS read size. (4-8K)
>
> Stephan
>
> (1)
> Fragmentation Considered Harmful (1987)
> Christopher A. Kent, Jeffrey C. Mogul
> WRL Technical Report 87/3

I've changed the default NFS read/write size to 8192 bytes in /sys/arch/i386/include/param.h, recompiled the kernel and work as expected by now.

(The NIC is a NE1000...)

Thanks.