I am connected to a wireless network using the Linksys WMP54GS network adapter (version 1.1, found that out when I looked for a driver and found that the 1.4 on my adapter's label was a misprint). The signal strength almost always says it is "excellent" (only sometimes do I get a "limited or no connectivity") but the speed always fluctuates between 11.0 Mbps and 36 Mbps (sometimes it goes down to 1.0 Mbps). The actual effect this seems to have on my browser usage seems inconsistent. Sometimes a graphics heavy page might take a little longer to load but a text heavy page might just quit after a couple seconds with a "page load error" (though I've only noticed that with the new Firefox browser so maybe the newest version of the browser itself is still buggy). I'm in the room next to the one with the computer hooked up to the router so I don't know why the speed fluctuates. All of the help topics on the Linksys support section seem to relate to fixing the signal strength which is not the problem.
I don't know how much other info you need but I did access the router's web configuration page and I changed the router channel and changed the administrator password from its default but the channel thing seems once again to only be related to signal strength.
Try TCPOptimizer to guide you on MTU settings in the router.
However, more likely is wireless interference. Scan for neighbouring wifi (Netstumbler is a more interesting alternative to the wireless adapter's own scan facility) and swap channels to about 5 stops from the strongest neighbouring signal.
Also relocate cordless phone base/video sender etc away from computer or router.