I guess you can say i am a Guru....
First and foremost does the wireless connection actually drop? Or does it stay connected but just fails to load up any webpages and in general just move slow?
If the connection actually drops off completely from the router...chances are the issue is going to be within the router or the wi-fi card itself..but if the xbox stays connected...it could very well be the wi-fi card.
If it moves slow/fails to load webpages here are some things you can try...
-Unplug both the router and the modem for about 30 seconds then plug them back up...sometimes it will get out of "sync" and not throw out internet (but if this doesnt happen on the xbox...this probably wont work)
-Go to the command prompt "cmd"...(if you are running vista or 7..make sure you right click cmd and click run as administrator). Once you are there go ahead and type "netsh winsock reset" minus the quotes....hit enter and it will tell you that the pc needs to be restarted, do so. any better?
-Sometimes the driver will bug out i have seen it countless times...in this case go to the device manager (right click my computer,properties,device manager)..look for network adapters and click the arrow to drop the menu down. Look for your wireless card right click it and hit uninstall..but do NOT check the box that says remove driver. just uninstall it...then on the top of the device manager window click the button that says scan for new hardware. It should reinstall the network card. connect back to the router and see if things are better.
I'm too lazy to type more lol. Try that stuff and let me know if anything changes. if not ill recommend a few more things.
Hope this post was useful
-GeRm