odd VPN3005 behavior

Greetings,

I've got a Cisco VPN 3005 running 4.0.3 code. It has an odd problem 
that the public interface will stop responding every few hours. If I 
check on the interface status it says it's up but won't ping in or out. 
If disable/enable the interface it comes back up.
Any ideas on what could be causing this? Anything on the config side 
I'm totally missing or perhaps an actual hardware issue that needs to 
be addressed?

Thanks for any help!

Lonny