Why does TC lose internet connection?

I recently bought a Time Capsule (3TB model) and about once a day it loses its connection to the internet.

I upgraded the software to 7.7.2 as prompted when I got it, so that's current, and I put it in bridge mode also as prompted. The TC is behind a Cisco cable modem provided by the cable company. When the TC loses internet access the cable model lights are still all green. I've tried restarting both the TC and the Cisco modem, no luck, the problem comes back.

Before getting the TC I never had this problem - I was running two wireless APs - a Netgear and ASUS and they were rock solid. Of course once or twice a year the cable company had an outage, but that's not the problem here.

Also, out of curiosity, I don't understand why the double-nat issue is a problem. With the Netgear and ASUS I had both of them routing and had no problems - WAN was 192.168.0.1, ASUS WiFi was 192.168.1.1 and Netgear was 10.0.0.1 - no problems ever. Why can't TC handle this too?

So, why would a $400 Time Capsule be flakey like this? When it works it's great, but it's pretty lame to randomly drop internet access once a day or so for no reason.

Any suggestions? Googling hasn't produced any results.