My AirPort started doing the same thing, and a few days later, completely stopped working.
I just replaced the PSU and updated firmware, but the flashing yellow light with warning came up for a moment.
I've cleaned the fan and it is working perfectly.
I suspect there is a firmware/software issue that Apple hasn't identified yet.
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=== Update (12/21/2016) ===
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I replaced the PSU and cleaned i did a factory reset and re-updated, but all to no avail.
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It now works OK for a time, then signals overheating, then the fan revs up to full speed which then cools it down. This cycle repeats. I also separated and enlarged the vents for in-coming and out-going air, so perhaps it's just a buggy example.
My AirPort started doing the same thing, and a few days later, completely stopped working.
I just replaced the PSU and updated firmware, but the flashing yellow light with warning came up for a moment.
I've cleaned the fan and it is working perfectly.
I suspect there is a firmware/software issue that Apple hasn't identified yet.