SOLUTION FOUND FOR MEDIA BRIDGE
To make media bridge stable, I got the solution in another thread.
USE FW 380.66 on AC66U (I used Merlins - the last one in 380.66 series - it was called 380.66_4).
Have not lost connection and required reboot for months (august 2nd i changed) since i changed over and that was the ONLY change i made.
I do notice that internet somtimes lags 1-10seconds, but never disconnects. Could be external issues (DNS or likewise) or some other issue with the FW.
Regardless I have no more issues streaming movies, file copying from my NAS and so on.
No more router reboots every 3-4hours

- just 100% uptime
After that ASUS broke media bridge mode for good it seems like
Cheers
Boogie