I would not fault the router being on all the time. I have my router on all the time but never sense any problem with it. DSL are not very reliable and subject to electrical/magnetic interferences (light dimmer for example). If you don't think these are the problem then I doubt there is much you can do about it. But good luck.
Broadband connections in our Cambridgeshire village are very slow owing to the distance from the BT exchange, but mine is worse than many, with an average download speed of 700 kbit/s.
I have a reasonably new Netgear router and my question is, is it detrimental to performance to switch of the router when not in use for several hours, for instance overnight? As the speed seems to have worsened over the last few months do routers wear out and need to be replaced?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic