This doesn't seem like much of an update
You don't say?
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A crappy router that works perfectly with everything except for one single client?
OK, it was a crappy ISP-supplied router... but a brand new Airport Extreme isn't much better. A little, but not a lot. Sometimes it lasts for 20 minutes before dropping out. The problem is a combination of a saturated spectrum and a crappy chip. The crappy chip would work fine with no interference, the interference wouldn't be a problem if the chip was better.
Have you tried interference robustness on the Airport Extreme? It supposedly helps with that.