Hi,
So just be clear - your APs have been running for 5 years without a reboot or firmware update (which would reboot them)?
If that is true, I would strongly recommend updating the firmware to the latest version.
But i would at least reboot them. There could be memory leaks over time that might cause the issue.
I have had connectivity issues which cleared after rebooting the APs.
My question about locations was physical - was an AP in a different building or specific part of it having the issue?
If an AP is broadcasting all the SSIDs then if any AP is having an issue it would affect all the SSIDs broadcast by that AP