SB6121 Keeps Rebooting When Connected to Wireless Router - XFinity

I have an interesting issue with my modem rebooting when my home network is in a certain configuration. Until recently, we had Vonage as our phone service and had to keep its router as the first device in line after the modem. So the "normal" configuration is like this:

SB6121 > Linksys RT31P2 (Vonage) > Netgear WND3800 > computers

When set up this way, the internet connection is rock solid, getting 20 mb down/5 mb up.

I recently upgraded my Comcast package to one that comes with the Performance Pro internet, advertised at 75 mb down. In troubleshooting why I wasn't getting the faster speed yet, I also decided to get rid of the Vonage router and had the network set up like this:

SB6121 > Netgear WND3800 > computers

With this sequence in place, I am able to get the advertised 75 mb down (actually getting over 80 mb) but the internet connection is completely unstable. The connection is solid for aout 15 to 30 minutes, then the moden starts rebooting every couple of minutes. The minute I put the Vonage Linksys router back in between the SB6121 and the Netgear router, the connection is rock solid again. But I'm stuck with the 20mb download speed.

When the modem is rebooting, the logs indicate a reboot due to power reset, whatever that means:


I'd appreciate any suggestions on troubleshooting this. I don't have any hope that Comcast support will be able to resolve this without me first buying a new modem and/or wireless router.