Id chk with your isp and modem. Something isnt right...does the router work if you tirn off the wifi radio? If it still happens then, there is am issue upstream
Possibly with a configuration too...
My modem has 4 LAN ports. I tried to connect my other pc to the modem LAN and it works flawlessly.
I pinged a local website for 6 ms continously with direct modem connection.
I tried my other pc through the DIR-655 and got 1500-1800ms pings... Then I also tried to connect the PC that gave 6ms with direct modem connection through DIR-655 and it also lagged down to 1500-1800ms. Also connecting to the router's config lags very much. This problem has not been around always. For a long times (many months) the router worked fine.
I think, but am not sure, that it slows down because of the multicast iptv transmissions. Right after I started to use iptv as my main tv source the slow downs started again... Strange thing is that I've changed ISP between the slowdowns but before on my old ISP I did sometimes use iptv, no daily. Back then the slowdowns happened rarely. Now I use iptv daily, and the slowdowns are almost every day also...
I might try to connect all my iptv devices directly to the modem, thus bypassing DIR-655 router. But that takes some time because I have to punch through holes and wire ~20 meters of CAT-6 to reach the modem...
EDIT:
DIR-655 is reset to factory settings, only setting I changed is "enable multicast passthrough" because without that option my tv doesnt work (as I explained I use iptv).