> Any ideas?

No ideas, but I think I raised a similar problem on this list about two
weeks ago.  Similar setup, dialup line, good quality connections, but
repeatable NFS errors due to lack of MBUF space when doing a build
across the link.  It always failed at the same point in the MAKE.  My
question was "How do I increase the amount of MBUF space", as I thought
that might just fix the problem -- I suspected it has something to do
with assembly / disassembly of PPP packets into the higher-level
packets, since MBUFs are used for this purpose; but I didn't get any
further with this problem, and don't remember getting any replies.
 
--Terry Moore, Databook Inc.