Me too - I can reproduce it by running ppp over a nullmodem
on two ports of the same machine (Each interface on a different net).
Mostly when I transfer bulk data (ftp direct or large pages in Netscape
through a locally running Apache-Proxy) the line suddenly becomes quiet
for up to 30 seconds and then it continues.
I have seen "sending packet too large to self" from time to time.
Interestingly, when I tunnel my proxy traffic through ssh it didn't
show up - then again I didn't watch every time. I will investigate.
Another problem is that neither deflate nor bsd_comp worked for me without
"out of compression syncs" - bsd_comp just disabled compression,
deflate gave up.
Regards, Bjoern