Have you tried changing the Channel number? Most routers use "6" or "11" as the default so try one of these or, if already using one, try the other. Avoid an "Auto" setting and avoid Channels 1 & 2, as many other wireless devices such as mobile/cordless phones use the frequencies associated with these channels.
If that doesn't improve things, there may be a problem with the type of encryption used - WEP/WPA/WPA2.
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