asterisk users - Feb 2011 - PRI B-Channel restarting itself continually

On our live server, running Asterisk 1.4.23.1 and DAHDI-Linux 2.1.0.4. On
occasion (not too rare, happens maybe once every month or two), the PRI
and/or DAHDI will stop working properly and we'll get repeated messages
like this:

[Feb 25 05:17:22] VERBOSE[9511] logger.c:     -- B-channel 0/1 restarted
on span 1
[Feb 25 05:17:25] VERBOSE[9511] logger.c:     -- B-channel 0/1 restarted
on span 1
[Feb 25 05:17:27] VERBOSE[9511] logger.c:     -- B-channel 0/1 restarted
on span 1
[Feb 25 05:17:30] VERBOSE[9511] logger.c:     -- B-channel 0/1 restarted
on span 1

What's going on here? Is there a way to fix this or work around the issue?
Every unused B-channel already gets restarted every hour automatically by
Asterisk.