Add me to this issue - a 2860n+ with wifi disappearing, and this has happened for some time - at least the last 3 or 4 firmware updates.

I notice mainly on the 5ghz network, and suspect the wifi is stalling and rebooting itself.

Anyone that's seeing problems with the 5GHz wireless 'disappearing' periodically, check the router's syslog output - it might be the Dynamic Frequency Selection system detecting frequency usage and blocking the router from using those channels, which should be logged by syslog when it occurs.
Beyond that, try using channels 36, 40, 44 or 48, those are the only 5GHz wifi channels that do not require the use of DFS.

If possible, please contact the support team with your issues so that these problems can potentially be resolved in future firmware updates.