Internet drops on IPV4 while IPV6 stays online

Since December last year I have problems with my Comcast SMC router (using Comcast Business Class Internet)

Several times during the day my IPV4 connection to the internet goes down while my IPV6 connection is still up.

If its down, I can still ping any internet address from the diagnostics screen on the Modem and it reports connectivity just fine.

But if I ping from my PC behind the modem (aka behind the Route in the SMC Modem), the ping fails.

That tells me that the SMC modem does not route properly anymore (aka is BROKEN).

So far non of the Comcast support engineers I tried to tell this understood what I am talking about.

Is there anybody out there who has the same experience?
How can I convince Comcast that there is a problem in the modem?

I had them exchange the modem in december but the new one does exactly the same problem.

To me this points at a Firmware issue of the modem.

I cannot use the Netgear modem as that modem is NOT able to turn off the IPV6 DHCP server in the modem and screws up my internal network (for some strange reason IPV6 DNS has priority over IPV4 DNS and I have my own DNS server for my inhouse PCs).

Does anybody have an idea what to tell Comcast or how to fix this?