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I've recently come to the nearly final conclusion that the hinge in the MacBook is affecting the AirPort card. A loose wire or a loose connection somewhere is likely at fault, because when the screen is in a range of about +/- 20° from a right angle to the keyboard portion, the wireless signal seems to work.

It could also be one of the many software things I tried to fix, but for now Carey's computer isn't going to sleep and it's working out well.

Update: This comment says: "You might have one of those models where the airport card is inside the cllutch/hinge of the display panel. Those machines are quite sensible to misplaced flex cable connectors…"

The comment goes on to talk about booting off an external system or using Target Disk Mode, but since it has no Firewire, that ain't happenin' any time soon, unfortunately.