WAN Interface loosing the IPv4 gateway

[vg8open]

I have a pfSense firewall version 2.1.4-RELEASE (amd64) built on Fri Jun 20 12:59:50 EDT 2014 with Realtek 8111E, and sometime the gateway is missing from the WAN interface.  Here's the log showing the errors.

Aug 5 23:00:32 php: rc.filter_configure_sync: Could not find IPv4 gateway for interface (wan).
Aug 5 23:00:32 php: rc.filter_configure_sync: Could not find IPv4 gateway for interface (wan).
Aug 5 23:00:32 php: rc.filter_configure_sync: Could not find IPv4 gateway for interface (wan).
Aug 5 23:00:29 php: pfblocker.php: [pfblocker] pfblocker_xmlrpc_sync.php is starting.
Aug 5 23:00:29 check_reload_status: Reloading filter
Aug 5 23:00:29 check_reload_status: Syncing firewall
Aug 5 23:00:00 php: rc.filter_configure_sync: Could not find IPv4 gateway for interface (wan).
Aug 5 23:00:00 php: rc.filter_configure_sync: Could not find IPv4 gateway for interface (wan).
Aug 5 23:00:00 php: rc.filter_configure_sync: Could not find IPv4 gateway for interface (wan).
Aug 5 23:00:00 php: rc.filter_configure_sync: Could not find IPv4 gateway for interface (wan).
Aug 5 23:00:00 php: rc.filter_configure_sync: Could not find IPv4 gateway for interface (wan).
Aug 5 23:00:00 php: rc.filter_configure_sync: Could not find IPv4 gateway for interface (wan).
Aug 5 23:00:00 php: rc.filter_configure_sync: Could not find IPv4 gateway for interface (wan).
Aug 5 23:00:00 php: pfblocker.php: Starting pfBlocker sync process.
Aug 5 22:45:00 php: rc.filter_configure_sync: Could not find IPv4 gateway for interface (wan).
Aug 5 22:45:00 php: rc.filter_configure_sync: Could not find IPv4 gateway for interface (wan).

I can still access the internet, but I noticed that the webpage is slow to load or timed out half of the time.  After I released and renewed the WAN IP address, the WAN interface shows the gateway and everything works good again.  Does anyone have any idea as to why this is happening?

thanks,
Brian