I had a bizzare problem with my router recently. I switched from Sky (too expensive, plus I dislike Murdoch) to Tesco (cheap, though I dislike their empire-building) for broadband.
My PC, a top-of-the-range Dell with an external aerial, is sat upstairs about 4m from the wireless router kept near the phone socket downstairs, in a modern house primarily made out of cardboard and gaffer tape. I never had a problem with the Sky router - got a solid connection all the time. Problem was, the Tesco connection kept "falling over" intermittently, and neither moving the router or the external aerial made any difference. My wife's laptop connection was always good, wherever the laptop was placed.
Eventually I rang the helpdesk, and - after a long series of conversations - they suggested it was interference with nearby (other houses') wireless networks. So I logged onto the router, changed the channels, disbelieving this could make any difference at all, and now it's solid as anything.
So - before coughing up any money - do try changing channels. You never know.
Andy