How to troubleshoot wifi devices that stop providing a signal

I have 15 Dlink DAP-2590's in our building - most of which work flawlessly.   I have 1 hallway however where they (2 of them) stop providing a signal periodically, bi-weekly or monthly.  This has been happening since they were newly installed.  I thought for a good while that it was signal conflict with each other - I put them on different channels, I moved them further apart, and finally I have turned 1 of them off and then the other but the problem remains even when there is only 1 of them and no other wifi within 300 feet.

I can see the ssid even when it isn't providing signal and restarting the wifi from the gui interface always solves the problem for awhile.

What troubleshooting can I do to solve this issue?  I hesitate to replace the wifi unit because it happens to 2 of them and surely the odds are not in favor of that being the issue.   I suspect some other interference but there are no other odd electronic devices in that hallway - no different than any other hallway.  There is 1 macbook air in the area...can a laptop cause interference for a wifi device?