On MacOS you don't have CyDoor but the java banners, right? CyDoor causes problems with the IE on windows systems.

On Linux, I'm having a problem with lock ups as well. There seems to be a serious bug somewhere, however I didn't even come close to verify it, yet.

My current guess is, that it's somehow connected to the requerying or downloading mechanism, since it seems to happen more often, when I have lots of downloads.