Shutdown -h does not complete

[James McKenzie]

Here is one for the books....

If I startup my system and immediately shut it down, it does.

However, if I bring up the ppp0 interface and drop it, shutdown -h or 
shutdown -r appears to hang shutting down IPTABLES.  This recently 
started and before this happened, I received the following error:
Cannot map key 255 to event 2072 (this is from memory) and the system 
will shutdown or reboot.
This is perplexing as IPTABLES should not be a problem as I can manually 
stop and start this process.
Any ideas why this is happening and what I can do to fix it?

James McKenzie