I always seem to be complaining about this at this time of year.
Installed a brand new Asus RT-N66U router on Sunday evening to replace my Billion 7800n that I was using with my Zen 80/20 FTTC, which was installed 16th July.
Since then my download speed has gone into freefall looking at my Zen Line Data and the BT Performance Tester agrees.
Could this be a fault with the router? Can see how as it is connected after the modem and has nothing to do with the incoming line.
I did notice when I was setting it up that if the router was sitting next to the modem, where the old router sat, and about 1' away from my wireless keyboard/ mouse receiver, my keyboard/mouse would not work, don't know if this is normal or not. So I ended up putting the Router up on the top shelf of the desk and 2' away from any of the other equipment.
Anyway tonight I have reconnected my Billion back up and it is all sat back in it original place. I will reboot the modem in the morning, if it will make any difference like ADSL. So will now wait and see if it gets better.
So if it was caused by the router, how long before it starts gaining speed again?