Root-NFS: Server returned error -5 while mounting /diskless

[mescall2000]

I have a annoying problem with PXE network boot.

I have 1 server and 6 terminals without hard disk/cdrom/floppy.

When I try to boot one terminal at once (for example start one terminal every 5 minutes) all work.

But if i boot all terminal at same moment, or 5 seconds different (the time to pass trough the desks and press the power button) only one X terminal appears, and all other freeze while try to boot with this error message:

...
...
Uncompressing Linux... Ok, Booting the kernel.
Root-NFS: Server returned error -5 while mounting /diskless_gentoo
VFS: Unable to mount root fs via NFS, trying floppy.
Kernel Panic - not synking: VFS Unable to mount root fs on uncknown block (2,0)

I ckecked some RFC and the error described are this:

NFS3ERR_IO
I/O error. A hard error (for example, a disk error)
occurred while processing the requested operation.

But the network cabling and hardware are fully operational.

Some suggestions?