Mike
If you've got a BT Voyager (205) router you might have heat problems - I did, and wrongly assumed slowdown/connection problems in the late evening were due to internet traffic, the US waking up etc, but they were actually due to it heating up after a few hours running (neighbour's connection was ok). Try your hand on top of it - hot to the touch. One website recommends placing a metal object on top as a heat sink.
Mine eventually stopped working.