CentOS VPN Box Neighbor Table Overflow?

I'm having problems with a linux box we're using as a VPN/IPtables firewall. The setup is such that we're sitting this thing behind a basic Comcast cable modem will all the pass through options set. The outside facing interface is set to DHCP and grabs an IP address from Comcast. Inside interface is statically set with the same being said for the CIPE interface. The inside interface works until we plug up the outside. When we plug up the outside interface, all network traffic to the box effectively shuts down and the /var/log/messages is flooded with "neighbor table overflow" messages. Sometimes, this takes a few seconds to appear, and when that happens I can track some traffic hitting the far side of the tunnel, I.E. it works for a few fleeting seconds.