mn-500 repeats connection-disconnection with ISP

I just got a new mn-500 and connected it with a dlink
cable modem. My ISP is comcast. After installation, i
downloaded the latest auto-update software and updated the
router (for both firmware and the management application).
The problem is now it keeps connecting and disconnecting
with the ISP frequently (a few seconds). Seems it keeps
trying to connect with IPS to get an IP and only few
seconds later, it disconnects with the ISP and tries to
get an IP again. When I ping www.yahoo.com, it usually
drops 50% packets and some times it takes long time to
resolve the IP address for an URL. I tried to replace it
with my dlink router, it works fine. I also tried various
setting combinations (enable/disable DHCP, firewall,
wireless acess ect.), and the problem is still there.