WP7601, losing IPv4 address shortly after connecting

After setting up a connection to Verizon, i successfully get an ipv6 address and ipv4 address. After a short while (few minutes or less), I lose the ipv4 address when checking ifconfig. Not sure why.

Has anyone run into this?

If the connection is idle, network can disconnect data session. If you keep a ping going, does this happen?