After replacing my old Telecom TG582n modem/router with a Spark HG659b modem/router to see if this would solve issues I was having with random ADSL disconnections, I am now having problems with the wifi speed. The ADSL issue turned out to be a cabling problem, but I decided to stay with the newer modem as it has gigabit LAN ports.

Periodically from once every couple of days to a few times a day the Internet speed (using speedtest.net) drops from 11MB/s to 3MB/s and the ping goes up from ~18ms to ~100ms. This occurs on both wifi connected Android and Windows devices. Devices connected via the LAN or another access point remain running at 11MB/s and ~18ms. Local wireless speeds are not affected ie I can copy a 100MB file from a Pc connected to the LAN to a wireless laptop in a couple of seconds. When the Wifi/Internet is slow the first hop to the modem/router is 2ms and the second hop to  adsl.bbspark.co.nz is ~100ms. The first few times this happened I reset the modem which temporarily fixed the problem, however I have discovered that if I make a change to any of the Wifi settings on the modem and Save this will also fix the problem immediately (until it happens again!). 

There are only 6 devices connected to the modem router, and the other Wifi access point is 100m away in a metal shed. I am out in the country so the next nearest wifi would be at least 500m away and not detectable on a wifi scan. The HG659b modem is on Channel 1 and the shed AP is on Channel 11.

The HG659b is running version V100R001C227B022 which I believe is current from other recent postings.

Any ideas appreciated, otherwise I will most likely switch back to the TG582n as before the cabling issue this was rock solid and ran without any problems.