Just had my RT-AC88U stop responding to all network traffic out (LAN and WAN) of the blue. I was streaming a YouTube video over wifi on my iPad and I noticed it started to stutter a bit and then stopped playing altogether. At that point all wired and wireless devices lost access to the Internet and none could ping the router itself. I tried pinging from the WAN side and that had no response either. Basically the router wasn't responding to any network traffic. I had to power it off and on.
I upgrade to 380.66_4 on May 27th early morning and up till it died it was fine. I checked the log and there was nothing between May 27 and the time it booted up after I powered it on.
I've read a few other reports of this happening to others in the thread, so it seems likely there's a bug in this firmware.
Of note I ran 380.65_2 for several months without issue. I ran 380.66 for about a week before I had to reboot. Similar with 380.66_2.