Why ? - first tell me why NFS with 4K packets is working beautifully 8)
Over ethernet the Linux kernel code will buffer up to 100 frames so it
shouldn't be hitting any limits (since it will buffer a full 64K frame
if need be). On the receive side your network card has to be able to
catch all the fragments which except for old old cards (ne2k, wd.c with
8K of ram) should never be a problem (even an NE2K shouldnt have a
problem except odd packets now and then).
It could be a kernel problem, but Im not quite sure where yet
Alan