[c-nsp] Strange ICMP/GRE problem

[Roy]

This one has me stumped a bit. 

I have a 1721 with two ethernet ports running 12.3(18).  The Internet is 
connected to the Ethernet WIC.  It has NAT enabled and a GRE tunnel.

In the failure mode, the tunnel drops and the router stops responding to 
pings from the Internet.  However you can still telnet into the router 
from the Internet. 

The log shows no events when this occurs either.  The ICMP counters in 
"show ip traff" seem to be incrementing as expected.

Any pointers would be appreciated.

Roy