< Thanks for pointing out this bad configuration.
  There is no homogenous network at our university so with three
  different login's this trouble can happen. >

I'm on a quite busy network and there might be a problem for the
server to answer just in time. Somewere I read about a 'delay constant'
of 15 in one of the files of 'sys' that had to be set to make a
freeBSD behaving as expected when booting a diskless PC.
( I have to look for it again in the WEB-pages )

Does the configuration look right if I set up all the config
files in the right way? (i.e mountd,nfsd,rarpd,rpc.bootparamd,rbootd and
the files ehters,hosts,bootparams,exports)

My server reports also quite often arplookup errors of addresses
which are just dummies or which harwareaddresses can not be found out.

And sometimes even le0:  reports some droppings of something (
It just does not happen right now so I have to look and find out
what it exactly says)

All in all it looks as if my servers network connection is not
set up in the proper way (although I can do NFS,ftp,rsh,telent and other
things without problems or lossesa).