I had a VZB T3 terminating on a 2851 with NM-1T3/E3, sufficient mmory and an AIM-VPN card.
T3 came up fine after initial installation, however, after a few minutes, performance degraded significantly. There were no errors on serial or controller. VZB also checked out line clean on their side.
After rebooting router, the same behavior was seen. Right after reboot, it works fine but after a few minutes, it slows down significantly. Again, no errors on serial or controller.
The same card on a Cisco 3725 does not have any problems.