My AC 87u has also been doing this for several versions 380.57 onward. The log for me looks like this:
Mar 9 17:27:40 miniupnpd[1319]: upnp_event_recv: recv(): Connection reset by peer
Jul 31 19:00:11 syslogd started: BusyBox v1.20.2
The event before the crash varies, and is hardly ever the same. Happens about 3 times a day and is driving me nuts!
Mine doesn't even get that far. It locks up hard where even the router ip does not respond to ping and the web interface is no longer accessible. i'm trying to setup a syslog server to log to, but for now i'm back to 56_2