Erratic TV Reception with DVR behind Router behind 3801HGV

I recently added a new router (Asus RT-AC66U) behind my 2Wire 3801HGV residential gateway by following SomeJoe7777's *very* helpful post:

(http://forums.att.com/t5/Residential-Gateway/U-verse-for-BUSINESS-2Wire-3600HGV-bridge-mode-or-another-AT-amp/m-p/2707755).

After doing so, Internet and all devices worked fine with the exception of my DVR - a Scientific Atlanta ISB7500).  With it connected to one of the new rtouter's ports, I was able to verify that it acquired an IP address from the router (which is on a different LAN segment than the RG); however, after about a minute, the DVR reported loss of signal.  Switching to another channel restored operation for about a minute, after which signal loss was reported again.
I found that I was able to restore TV operation by plugging the DVR into one of the RG ports, thus bypassing the router.  For several reasons, I would like to have everything (including the DVR) on the new router.

Any thoughts on what I need to do to accomplish this?

Thanks.