[c-nsp] Ignored input errors on Fa interfaces

[Ras]

I have a 2811 deployed at a customer site, with Fa0/0 facing a 100M
circuit back to me and Fa0/1 facing the customer.

Each interface is clocking up (albeit at varying rates) a bunch of
input errors that are 'Ignored':
FastEthernet0/0 is up, line protocol is up
     7 input errors, 0 CRC, 0 frame, 0 overrun, 7 ignored

As far as I can tell the interfaces aren't overloaded (~3M peak),
there are plenty of buffers to go around and the CPU/memory seem
happy.

This router really does very little, although it is multicast routing
but again only 2-3M peak.

I've read the Cisco docs on debugging these errors but havn't been
successful in eliminating them, including after a reload.

Does anyone know how to dig deeper into this / find out what is causing this?

Thanks,
Ras