Wireless connection drop offs

I am having trouble with my wireless connection. My pc will connect for anywhere from 3 to 5 minutes and then suddenly drop the connection saying no wireless network found. then after several minutes, it finds and reconnects only to stay for a few minutes and drop off again.
My equipment is listed below: Thanks for any and all help!!

The modem/router is a motorola SBG900 wireless surfboard Gateway. I have the internal firewall disabled. I am using WPA security.

I have my primary PC hooked to the modem with RJ45 cable and I have no problems with it.

My 2nd desktop PC is a Gateway 2K 1.2GHz AMD running Win XP Pro (SP2) with 896 MB RAM
I have a US Robotics USB wireless adapter Model 5421 plugged into USB 2.0 port. I have downloaded and installed the latest drivers from USR website. I have tried using both the WCZ service and the software that came with the adapter. I get the same result with both. I have also checked to make sure the 802.1x Authentication box in the config properties is unchecked (based on info from a friend).
For further info, the computer is sitting in the same room with the router so signal strength is not a problem.
I appreciate all the help I can get!!
Thanks for not replying with "get a new wireless card or modem" responses, I have to play with the toys I have. I am on a budget...

 

Unfortunately I wouldn't be able to give you a solid answer. However you need to systematically eliminate possible causes until you narrow down the possibilities.

You can start by completely uninstalling the drivers for the wireless card and of course reinstalling them. I know your on a budget but somehow if you can get your hands on another wireless adapter, or a laptop with wireless (you can borrow from a friend). That way you can test the connection and determine if it is the adapter or the router/modem. It seems as if the router/modem work when connected physically so I would begin with the wireless adapter.

There may be a firmware upgrade from the manufacturer of the router/modem available. I am assuming that there is no current warranty?

On the bright side at least you still have a wired connection for research (google) purposes.