ISDN connection problems

[Stefan Pietsch]

Hi,
I have a FreeBSD ISDN router running 5.3 with an AVM Fritz card and
a 3com etherlink xl. After having mastered the xl driver problem
(http://www.freebsd.org/cgi/query-pr.cgi?pr=68435) I noticed that the
ISDN connection breaks randomly after some time.
The PPP daemon stays up, but the ISDN interface does not send any
packets. A ping to an internet address shows the following:

ping: sendto: No buffer space available


After that the only thing I can do is shutdown PPP and dial in again
to get the internet connection working. This is really annoying.
I haven't found a solution yet.