There are lots of things you could try.
The simplest thing is to try placing your router as high and centered as possible.
You can also use Google and search for something like the following:
getting the best wireless signal
I personally use a wireless bridge setup.
Am I right that I can change the channel on the router's website. Related to that, I know that the router id and password were "password1" upon installation (last week). I logged into it myself several times. Just yesterday I noticed this wasn't working. Does the id/password reset itself to something? This is very odd.