The earlier versions of the Airport cards had problems when hot. I don't know if this was your problem, but if your wireless connection was generally stable when the laptop was cool, and then degraded with time, this might be the reason.

Also, some WiFi cards perform a self-calibration only when initially powered on. As parameters drift with temperature (and perhaps with supply voltage), the card may misbehave. If simply powering off and then back on again restores stability, then your card most likely implements that sort of calibration. I do not know if the Airport card works this way, but if it does, that could explain a few things...