Ok, that doesn't prevent an NTP issue. You've described the physical layer: 3 hard wired WiFi Access Points. That doesn't mean there can't be a problem with the physical layer (I've seen sources of interference cause problems on cable before, could be solar panels or a noisy misbehaving electronic device). Could also still be NTP, just because they have a physical connection does not mean UDP NTP traffic works reliably. This specifically could cause a problem with authentication failing, and seemingly at random. Be sure they have a good time source in their configuration. Could also be that the client doesn't have a good time source. Could also be heat, is the AP in question getting hot? Is it near a heat source? What about its power supply? Could also be hardware failure of the AP itself.