With .11g, you'll also see it drop back to slower speeds if there is a slower device on the air, such as a PC with a .11b card in it. Since it is a shared medium, all connections will suffer.
Also, increase the buffers in the player if you can. It'll take longer to start up the playback, but it should be a bit smoother if the network is choppy. I also wonder if there may be a problem with the way the file is stored - could the file compression be causing poor throughput on .11g ?? You can test this with different file formats that offer different compression levels.
I'd test here, but I don't have a wireless PC now - my wireless is shut off at the moment because there's nothing to support.
Are you sure it isn't the server's LAN connection?