Trying Kong Mod FW to resolve ipv6 unreliability with stock fw

Hi.  Got a new R9000 and was running with latest stock firmware 1.0.2.32.  ISP is Cox (Orange County) via Netgear CM1000 cablemodem running v2.01.16.

After 24-36 hours of uptime, the router would no longer communicate in ipv6.  Appeared to be fine, DHCP provision of ipv6 info to clients looked fine -- but no ipv6 throughput.

Rebooted cablemodem first to try to narrow down issue.  No joy -- until reboot of router then good again for a long period of time.
 

Just installed the current Kong Mod.  ipv6 set to DHCP with PD, and other than custom DNS all else defaults.  MTU set itself to 1452.  Seems to be working great.
Any ideas why my ipv6 was unreliable on the stock firmware -- and can I be hopeful the Kong will do better in this regard?  Lots of discussion about ipv6 problems with various routers and various isp's... hard to believe that ipv6 "auto configure" /  "DHCP" isn't all routine by now.

Thanks! /jim