Netgear WNR854T Loses WAN IP

I've had the above router for several months, and it has worked
flawlessly, until about 2 weeks ago. Since then, when we're on the
internet, sometimes when we're downloading video or other large files,
the router loses the WAN IP address. The LAN IP is not lost, and I can
connect to the router wirelessly when we lose the WAN IP. I contacted
Netgear support, and they had me change the MTU values and re-install
the latest firmware version, which was already installed, but those
didn't help. They decided the router must have gone bad, so they set
me up for a replacement. To cover the time while the old router is in
transit to Netgear, I purchased another router of the same type. Much
to my dismay, it exhibits the same problems. Any ideas what might be
going wrong?

My ISP is Time Warner Cable/Roadrunner w/ a Dynamic IP

The router's firmware version is 1.4.23NA, which is the latest.

Thanks for the help.

Joel