Ik heb nog wat verder gezocht en het is dus kennelijk een known issue:
Dit las ik op macpod.net (ik heb overigens geen Intel):
News
October 23, 2006 - The Wireless Grapher Widget should no longer cause connectivity issues. The problem seems to be related to calls to the WirelessScan() and WirelessScanSplit() methods. When these methods are called under 10.4.8, the airport extreme card in intel macs seems to be unable to juggle maintaining the current connection and identifying other networks in the area and consequently severely slows down network connections or even disconnects! It wasn't like this in 10.4.7.. I wonder what Apple changed...
October 6, 2006 - Two users have reported connectivity issues with their Macbook Pro's running 10.4.8 and the Wireless Grapher widget. If you have encountered such problems remove the Wireless Grapher widget from your dashboard and then reboot your computer. If this happens to fix your issues, please email me so that I may work with you to provide a fix.