Brett Tiller:
What IOS release are you using, and what is your router platform?
15.3(1)T and 3925.
Brett Tiller:
Apparently this issue only affects VPN packets? In other words if you were not using VPN, this issue does not occur?
Correct, we just verified this by removing tunnel protection from the tunnel interfaces of the affected router and the head end router and the packet loss went away. Nothing else was changed anywhere in the network. Maybe a very odd interaction between crypto and contention to the backplane?