Occasional 100% CPU on 2811 doing T.37 fax

Question

Hi,
We support many 2811 gateways at customer sites, all handling T.37 fax (almost entirely onramp) and nothing else.  Occasionally we see bursts of the following in the logs, showing that incoming fax calls are being rejected due to high CPU:
Sep 12 12:07:14.980 CDT: %IVR-3-LOW_CPU_RESOURCE: IVR: System experiencing high cpu utilization (96/100).
     Call (callID=1365621) is rejected.
Sep 12 12:07:20.864 CDT: %IVR-3-LOW_CPU_RESOURCE: IVR: System experiencing high cpu utilization (96/100).
     Call (callID=1365622) is rejected.
Sep 12 12:07:50.564 CDT: %IVR-3-LOW_CPU_RESOURCE: IVR: System experiencing high cpu utilization (98/100).
     Call (callID=1365625) is rejected.
In the above example, the logs continued for about 7 minutes.  A 'show processes cpu history' was done about half an hour after the event and showed this in the 'last 60 minutes' graph:
                                 11111111                      
                           1111  0000000099988777666554433332111
    347444468456468864435765010640000000084064942720627596224963
100                              *########                     
90                              *##########*                  
80                              *#############                
70                              ################*             
60                              ###################*          
50                              #####################**       
40                              ########################*     
30                              ###########################   
20                        *     #############################*
10   *    ** ** *##*   ***####* ###############################
   0....5....1....1....2....2....3....3....4....4....5....5....6
             0    5    0    5    0    5    0    5    0    5    0
               CPU% per minute (last 60 minutes)
              * = maximum CPU%   # = average CPU%
Most recent CPU is on the left, so this shows that the CPU was ramping up over a period of 20-25 minutes, remained at 100% for about 8 minutes, then abruptly dropped back to normal.  Uptime was 30 weeks when this happened.  Because it's so infrequent and we have no way to reproduce it, it's unlikely that we'll be able to grab diags while the CPU is at 100%.
The IOS version is 12.4(25f), running on a 2811 with the NM-HDV2-2T1/E1 board and the PVDM2-48 DSP card.  The DSP firmware is overridden to 26.4.501 due to previous issues with out-of-spec tones and outbound fax not working.  It has 512MB memory and 128MB flash.  I see there's a 12.4(25g) IOS version available, but the caveats list doesn't seem to contain anything relevant.
Any ideas as to what might be causing this?  A rogue fax?  Is IOS 15 likely to help?  Some of the fax gateways we support only have 256MB memory so won't be able to go to IOS 15 easily.
Suggestions appreciated -- thanks in advance.

Answer 1

Try following cmd's , then check the interface errors.
network-clock-select 1 BRI 0
network-clock-select 2 BRI 1
Try this doc:
http://www.cisco.com/en/US/tech/tk652/tk777/technologies_tech_note09186a00801fe5d9.shtml
http://www.cisco.com/en/US/tech/tk652/tk777/technologies_tech_note09186a00801fe5dc.shtml