asterisk users - Apr 2009 - random hangups: how to debug?

I've a TDM400 with dahdi 2.1.0.4, asterisk 1.6.1-rc5. Asterisk is 
randomly hanging up calls coming over the pstn. Often it happens right 
as the call is answered:

     -- Starting simple switch on 'DAHDI/4-1'
[Apr 22 17:09:38] NOTICE[20123]: chan_dahdi.c:7505 ss_thread: Got event 
18 (Ring Begin)...
[Apr 22 17:09:40] NOTICE[20123]: chan_dahdi.c:7505 ss_thread: Got event 
2 (Ring/Answered)...
     -- Executing [s at incoming-pstn-line:1] Answer("DAHDI/4-1",
"") in
new stack
     -- Executing [s at incoming-pstn-line:2] Wait("DAHDI/4-1",
"3") in new
stack
     -- Executing [s at incoming-pstn-line:3] NoOp("DAHDI/4-1",
""callerid
  :""" <>") in new stack
     -- Executing [s at incoming-pstn-line:4] Set("DAHDI/4-1", 
"CALLERPRES()=allowed") in new stack
     -- Executing [s at incoming-pstn-line:5] Dial("DAHDI/4-1", 
"DAHDI/1,60") in new stack
     -- Called 1
     -- DAHDI/1-1 is ringing
     -- DAHDI/1-1 is ringing
     -- DAHDI/1-1 is ringing
     -- DAHDI/1-1 answered DAHDI/4-1
     -- Native bridging DAHDI/4-1 and DAHDI/1-1
     -- Hungup 'DAHDI/1-1'
   == Spawn extension (incoming-pstn-line, s, 5) exited non-zero on 
'DAHDI/4-1'
     -- Hungup 'DAHDI/4-1'

but sometimes it happens after 2-3 minutes of conversation.

I figure this is a dahdi problem, but how do I get a more detailed log 
of what's happening here? Is there a way of setting up debug messages 
only for dahdi? Since it's random, I'd be collecting a lot of junk to 
just set debug.

sean