Dell 640m and BT Broadband

I have a BT wireless modem/router for broadband which is cabled to my PC but when I want to use net wirelessly on laptop...althoughtthe signal is good-excellent..it takes an age for a webpage to load initially. once it does load its grand then for surfing the net but if I leave it for a minute or 2 idle, it goes back to square one and can take up to 10 minutes to load a webpage again. Getting very frustrated with it now and wondering is there something I need to do or some setting to change?

Thanks

edit: its 2mb broadband by the way