I am running NetBSD-current on a PC attached to a SUN via
19200 baud serial cable.  Everything is wonderful except
NFS, which crashes after transferring 30-40K of stuff, and
the PC freezes.

I have a cheapo serial card with no fifo, and I see error messages
"3 silo overflows", where 3 is variable.  I conclude that PPP
is not error-resistant??? though I see ack/nak and error checking
code.

I'm running PPP-2.1.2 on the Sun side, and the distributed PPP on
the PC side.

Kermit transfers work fine (that's how the whole 200 megs of Unix
and X got onto the PC), though they're about twice as fast under
DOS as under UNIX.

So:

1) is there a recommended fix?

and 

2) if not, is there a recommended approach for debugging?