I was having a similar problem with firmware 5.0.2.
Tech support had me upgrade to 5.0.3 and so far so good.
Since I don't yet have a powerbook, I've been using the APBS simply as a router (in anticipation of getting a powerbook eventually).
Question: Is this firmware update meant to address the issue of WIRED machines dropping out, WIRELESS or both? I read the support doc and am a bit unclear.
And just to air a gripe: I wish the knowledgable guy at the Apple store had told me about the firmware update before I walked out with my AEBS purchase, considering the update had already been out a week or so. This would have saved me a lot of frustration.