Cisco bug ID CSCsb60583 documents this issue.
When you run a Cisco 2800 Series Router at high CPU utilization, the I/O memory begins to decrease until all memory is exhausted. At this point, the router begins to report Pool Manager tracebacks.
This symptom is observed on a Cisco 2811 or Cisco 2821 that uses the onboard VPN crypto engine. The symptom is prominently observed when crypto is configured over serial interfaces.
In order to resolve this issue, upgrade to Cisco IOS Software Release 12.4.10.
In order to download the image, visit the IOS Upgarde Planner.