Got 951G-2HnD, bought it with L4 license and 5.25 OS version.
After upgrading to 6.7 and firmware to 3.10 got frequent internet disconnects (wan port traffic dies in statistics) during high bandwidth utilization, (only reboot or 15 min of waiting helps, strange though) I've reseted config and made new manually, I've got simple AP mode with Nat and WiFi.
Wan port is pure Ethernet with dhcp, nothing in logs.
Upgrading to 6.9 didn't resolve my issue, resetting config and again making it manually again didn't help, l don't see anything strange in logs, traffic just stops on wan port.
Got no POE enabled, Had to downgrade to 5.26.
Repeat - this problem occurs only on 6.x versions during high bandwidth utilization - e.g YouTube FullHD or torrents
Anyone saw similar problems?

I am using the exact same router at a lot of locations and ROS 6.7 is what we currently run on most production units. We have never had this problem. However, I have had multiple RB900 series that needed a fresh OS install using netinstall. I don't know why, but that has cleared some of the bizarre problems that I have seen. I max out the RB900 I have on my Desktop most everyday and I haven't seen that problem.