I have been struggling with this issue aswell. It seems to only occur in PowerBook G4 machines. I have tested this with a MacBook Pro and it does not happen.IMPORTANT: To find your Wireless Transmit Rate, ALT-CLICK the airport menubar icon.Symptoms: When you connect to your wireless network, your transmit rate will be 54. (if you're using the 54g standard). If you start up a download or something that occupies your bandwidth (even LAN traffic), you will notice that the transmit rate drops to 11, then to 3, then to 1 until finally your downloads will drop, your ichat connections will fail and your browser will not load pages. My attempts at solving this: - Changed the wireless channel between 1 and 11. DOES NOT FIX THE ISSUE.- Changed encryption to shared key/open key. DOES NOT FIX THE ISSUE.- Removed encryption. DOES NOT FIX THE ISSUE.- Changed DTIM, RTS, Fragment length settings. DOES NOT FIX THE ISSUE.- Forced 11b mode. DOES NOT FIX THE ISSUE.- Forced transmit rate to 54Mbit/s. DOES NOT FIX THE ISSUE.- Downgraded the firmware to an earlier version. DOES NOT FIX THE ISSUE.- My access point is a Conceptronic C54APT. There is a hack that allows you to flash it to a D-Link DWL-2100AP. I flashed it and it DOES NOT FIX THE ISSUE.Solution:- I have another access point in the house (although, it does not reach my entire house, which is why I have the Conceptronic). It is a linksys WRT54G. I can CONFIRM that this issue does NOT occur with the linksys. It works great.- Just 30 minutes ago, I remembered that my access point has a hidden feature to boost the transmit power. I boosted it to 24 dBm and for the last 30 minutes, I downloaded over 1GB of data and was not disconnected however the connection is extremely unstable. The download speeds vary between 300KB/s and 1MB/s whereas it used to give me a stable 1MB/s in Tiger.- Before you tell me that the problem is my Conceptronic access point, remember that it always worked PERFECTLY in Tiger. (aswell as any other OS or device such as a PDA, a Nintendo DS and a Nintendo Wii)My opinion of what's happening: I believe this is because of the new self-tuning TCP feature. If you alt-click the airport button a few times during a download, you will see that Leopard adjusts your transmit speed so that a more stable connection is possible. This adjustment is what is causing the problem.