Try downloading NetStumbler
and running it on the wirelessly connected computer. Look for your base station ID and the strength it shows for your signal, then look for any other signals on the same channel or plus-or-minus three channels from yours that have signals near or greater than yours. So if you are on channel 1 look at signals on channels 1-3, if you are on channel 6 look at 3-9, and if you are on 11 look at 9-11. If you are getting interference from another WiFi station it should show up there. If you find something, try changing your router/AP channel to either 1, 6, or 11 depending on which has the least interfering signals in your area.
An interfering signal may be something other than WiFi, but these can be hard to track down. Common ones are radio based remote controls (not infra-red), baby monitors, cordless phones (in the 2.4Ghz band), and microwave ovens (usually interfere mostly with higher channels). From the sounds of it you are dropping often enough that it isn't likely to be the microwave or cordless phone (unless there is a teen female in the house). Finding this kind of interference takes more specialized equipment, the best bet may be to try another channel.
If the problem isn't interference it's probably one end or the other's radio going away, in which case it will get worse over time with no easy fix (other than a shopping day at NewEgg or your Techie shop of choice). A common fault is for the clock to drift and cause the rotating keys of WPA to rotate before the other end is ready. You can try changing to WEP or, if you like to live on the edge, turning off security on your router for a while to test.
Let us know what you find.
Edited by Orecomm, 17 March 2010 - 05:47 PM.