I would suggested that if ssh is the symptom, the problem is tcp.
This is highly unlikely; ssh (the protocol) has its own integrity
protection, so unless you have turned off MAC checking (or have an
implementation that doesn't do it, or has buggy code for it), ssh will
normally report an error rather than silently delivering wrong data.
sshv2, at least. I think sshv1 has something similar, but I don't
know
it well enough to say with any certainty.
Especially in view of Brad's report of seeing this on simulated
hardware, I think it's far more likely that there's buggy crypto code
lurking somewhere. It might be interesting to try moussh instead of
openssh, since it doesn't use the same crypto code; I should dig out
one of my desktop VAXen (I think I've got at least one) and see if I
can reproduce this.