I confirm this bug on 380.66_4 (n66u).
I think 380.66_4 and 380.66_6 have the same bug: the router restarts itself with no reasons.
My config:
All settings are default. Also I made reset to factory settings.
Dual WAN is configured in failover mode:
1 WAN: cable is not connected.
2 WAN: USB as...