My Asus RT-AC87R router was doing similar things and after trying everything under the moon I replaced it with a TP-Link AX11000. It's a little overkill for most but it's working great.
I also had issues with my Arris SB8200, time sync, over range, etc. I had Xfinity tech out twice, the 2nd tech noticed my modem only had one blue light on the front instead of 2. Luckily I still had my old Arris SB6190 so we swapped it and both blue lights came on.
