frequently drop from wireless network

I have similar problem with my D-Link 11b wifi router recently (after 2 years of operating just fine). Initially I thought that my recent upgrade to XP-sp2 was the reason. When I looked at the connection "status", with the wireless laptop only 10 feet away, I saw my signal strength goes up and down constantly every other second from full strength to none. Fortunately, Fry's had a sale of their Airlink 11g, and once I replaced that D-Link, all is well again. Still haven't figure out whats wrong with the D-Link. It is now just stting there in the box. So my suggestion is to borrow/buy another wifi router and try that, setting the SSID , WEP/WPA in the router the same as before so you wont have to do a thing to your wifi client/laptop.

The signal on my server is always excellent, but it will disconnect once in a while. The router is also my modem, so it is hard to get another one.

May be periodic interference. See the FAQ below. Try a different non-overlapping channel (1, 6, or 11). More the client much closer and see if that solves the problem.