I had a similar issue with Dir-655, Ping test to the router from PC was not loosing packets
but here and there I was getting upto 9ms return on a 1ft cable, also I have multiple PC connected
via LAN cable, One PC will do good while the other will slow down, updated the firmware switched cable but nothing helped. And occaisionally the connection will be just broken ... Reason BAD HARDWARE
Took it back and got a 825, works pretty good .. you should swap yours too